Ayutthaya: ancient capital. The old city has been kept very green, the ruins are surrounded by trees or greens. Really great. The city is quite large so it takes one day to see the interesting part (partly on foot, partly going to more remote temples with a tuktuk).
Lopburi: short visit to see the Khmer temples and the monkeys. The largest temple is in front of the train station, the other are within walking distance.
Kamphaeng Phet: really great, and there were no tourists, probably because all go to Sukhothai. It is also more cumbersome to reach if you arrive from the south (no direct buses from Ayutthaya, we had to go by train to Nakhon Sawan and take a bus from there (where train and bus station are quite far apart, but the collective taxi costs only 50 THB per person). On the other hand, Kamphaeng Phet is only 1 hour away from Sukhothai, so staying in Sukhothai and spending the day in Kamphaeng Phet would be quite easy.
The city is definitely worth a visit. The old city (surrounded by walls of which you still see some parts) has two nice temples remaining, with large Buddha statues. Most ruins lie to the north of the city walls. There is a series of temples in a forest, and walking among them, with noone around, was really great. The most well-known temple is Wat Chang Rob because of the elephant statues at the base of the chedi.
Sukhothai: the old city is some kilometers away from the new town, and there is a regular song-teo service. The site is incredibly large (so the fact that there are many tourists is not a problem), with lots of beautiful temples, and you can walk or rent a bike to explore it. For the most remote temples the bike turns out to be necessary.
Si Satchanalai: smaller than Sukhothai, with more forest around, and less visitors. Definitely worth a visit if you have time.
Lampang: great temple of a different style (Lanna culture, with many wood carvings).
Chiang Mai: the big city of the north. Nice city, many interesting temples. It is also a good place for shopping. The surroundings are worth seeing. Many agencies organize tours (trekking, elephant camps, visits to villages of the ethnic minorities...). Unfortunately the tribal museum was closed for restauration during our stay. It is situated to the north of the city, in a very nice park with a small lake surrounded by small bars/restaurants.
Ban Chian: archeologically very important site. Rather tedious to reach (bus from Udon Thani, then 6km of tuktuk). The site itself is not really interesting but the museum is really worth seeing. It shows the pottery found on site and has also many explanations about the region.
Phimai: great khmer temple, partly reconstructed, in the center of the city. The museum is also very interesting, with many statues and lintels found there and in the region. The city itself is very small and quiet, very nice.
Phanom Rung and Mueang Tam: two beautiful khmer temples close one to each other. The design is similar to Phimai. Phanom Rung is on a hill and offers also a nice panorama. Mueang Tam has also nicely L-shaped ponds.
Khao Yai: for nature and wildlife watching.
Phang Nga Bay: great scenery, with limestone cliffs falling vertically into the sea. James Bond's island is just one of many...
Railey peninsula: great beaches, especially Phra Nang beach...
In Bangkok, the simplest is to use taxis. There are many of them and they are cheap (they have a meter, you may have to insist on putting it on but it happened to us only once). Avoid the tuk-tuks which are in the end more expensive and who do not run faster given the heavy traffic. Metro/skytrain is good but the network is very limited.
Outside Bangkok: very often, there are no (or very few) taximeters. You have often to rely on the tuktuks. Prices must be negotiated beforehand. In some places, it is also possible to use the collective taxis which run on definite routes. Prices are lower but it is not often easy to understand where they are going.
Trains: can be very slow.... we took a 3rd class train from Bangkok to Nakhon Pathom (around 50km), it took 2 and a half hours (return journey by bus; 1 hour). We also took a 3rd class train from Ayutthaya to Lopburi (slow but almost on time), and a 1st class one from Lopburi to Nakhon Sawan. First class trains cost ten times more than 3rd class ones. They have the advantage that they do not stop everywhere but on the other hand the air conditioning was not working well and the seats were really bad. Third class trains have no air conditioning but then at least you can open the windows and get some air. Overall, train travel was not a very good experience.
Buses: the bus network is very good (roads are also very good). Avoid 'VIP' buses which are much more expensive and with which we had one of our very few bad experiences (the company was named Chan Tour): first of all, there were cockroaches on the bus... then, they were supposed to drop us off at an intermediate place but they just forgot! We had to get really angry to have them stop and then instead of bringing us back (only about 10km) they were waiting for another bus of the same company going in the opposite direction. In the end we managed to get a (3rd class) bus of a different company and to reach our destination, but it was not thanks to them...
Overall, service in buses (except the 'VIP') is very good. The lowest class can be slow because they stop everywhere, but at least they are reliable, and people are nice, will check that you go out at the correct stop, etc...
Planes: we took planes to go from Chiang Mai to Udon Thani (through Bangkok), and to go from Bangkok to Phuket and back. We used Thai Airways and Bangkok Airways. Planes were perfectly on time. Quality of service was excellent. In particular, we were impressed by the fact that Bangkok Airways provides lounges for ordinary passengers (no need to be first or business class, or frequent flyer) which are similar to the business class lounges of european companies, with free refreshments (and even a free Internet connection).
Boats:
-in Bangkok, the Chao Praya Express Boat is a nice and cheap way to go along the river. There are also boats crossing the river at selected places (for instance to go to the Wat Arun). We also took a long tail boat to go along the Thonburi canals and to the Taling Chan floating market.
-the tour in the Phang Nga bay was on a long tail boat. The only inconvenient is that they are very noisy.
-to go to Tonsai bay, the only way is by long tail boat from Ao Nang or Krabi. There is no fixed schedule, as they wait to have enough passengers (7-8) before leaving (or else you have to pay much more). We however did not have to wait very long (same for the return trip, where we however had to jump into the water -holding our bags- because there were too many waves for the boat to reach the Ao Nang beach, and the tide was too low for it to reach the more protected harbor...).
Bangkok:
Rambuttri Village Inn, 850 THB (no breakfast). In the Khao San Road area which is full of guesthouses. Clean and functional but very large and rather impersonal. Small swimming pool on the roof, which however closes rather early. Less nice than what it looks on the website, but still ok. Moreover, the area is rather convenient as you can walk to the city center.
Suk 11: slightly less than 1000 THB. Large guesthouse in the modern part of the city. Very reasonable, with a nice buffet breakfast.
Ayutthaya, Baan Eve: clean, comfortable, old style buildings.
Kamphaeng Phet, Chakungrao Riverview, see also this website. 990 THB, including breakfast. Great hotel, large room with nice view. The restaurant also provides great food at very reasonable prices.
Sukhothai, Lotus Village. 1350 THB, breakfast included. A bit more expensive than other guesthouses, but great place. The only problem was a peacock which was quite noisy even in the middle of the night. Lotus Village also provides tour services.
Chiang Mai, Sri Pat guesthouse: very nice guesthouse. People were extremely kind. The room (clean, spacious, quiet, AC) was supposed to be 1000 THB per night (no breakfast). As there was some noise during the day because they are installing a pool, we got a discount of 20%. Note that we had not asked for any discount, especially as there was anyways no disturbance during the night, and we would have been ok to pay the normal price.
Khon Kaen, Mai Thai guesthouse. 500 THB (no breakfast). Guesthouse recently opened by a retired englishman. Extremely comfortable accomodation; nice rooms decorated with taste, at a very low price. If you consider eating on the premises (as we had to do because it had rained so much that the roads around were flooded, and we did not want to go out for dinner), be aware that no Thai food is served, but only simple but certainly good English dishes.
Phimai, Old Phimai guesthouse. 850 THB (AC but no private bathroom) We went to this guesthouse because they organize the tour to Phanom Rung and Mueang Tam. The people are extremely nice but the accomodation, in a traditional Thai house, is a bit on the rough side (small room, no private bathroom). It is anyway possible to take the tour without lodging there.
Pak Chong / Khao Yai, Resort. Reserved for us by Greenleaf guesthouse and tour at a discount rate of 1000 THB (no breakfast; it seemed the normal rate was rather 1500 THB). Great room away from the busy road.
Phang Nga, Phang Nga Inn. The only problem is that they have no website and we did not manage to book by Internet. The guy from Sayan Tours however had very nicely booked for us. The Inn is very nice, with clean, quiet, well furnished rooms.
Tonsai Bay, Tonsai Bay Resort. 1350 THB per night in a great bungalow, including breakfast. Luxury at a budget price, close to the beach, and with wildlife around (such as macaques coming into our bungalow!). There are other accomodation possibilities on Tonsai Bay but this one definitely looks much better than the others (people go mostly to Tonsai to climb, not so much to enjoy the beach, and seem most often to be on a tight budget...). The beach is not the best of the peninsula, as at low tide it is rather rocky, but on the other hand the other beaches (Railey East and West) are much more touristic, resorts are one next to the other with much less space between bungalows. At low tide Railey West is just 10 minutes away. The best beach is in any case Phra Nang beach on which there is only one very expensive resort, Rayavadee resort. Phra Nang is reachable from Tonsai in 45 minutes by walking (quite steep trail through the jungle, enough to sweat a lot given the humidity...), first going in 30 mn to Railey East (not a very nice beach as there is mangrovia) and then 10-15 mn along the shore to Phra Nang.
If you want to spend more, you still can: check out for instance the Tamarind village in Chiang Mai or the Rayavadee resort on Phra Nang beach.
You also find a lot of fresh fruit: mangoes are wonderful, bananas have a great taste. Depending on the season, you will find rambutans or lichis. We discovered mangustans, a very strange fruit with a thick violet husk and white quarters inside. The taste is simply delicious. We also ate durian, which is well known because of its odour. Taste is somehow similar to caramelized onion jam. You find of course coconuts (and most importantly, coconut shakes!!!) everywhere.
Two particular restaurants that we want to advertise:
Tha Nam Restaurant in Chiang Mai. Very nice restaurant situated along the river in a traditional thai house. Northern thai specialties which differ from the usual food you find everywhere. Very good.
Benjarong Restaurant in Bangkok (in the Dusit Thani hotel). On the expensive side for thai standards, but extremely cheap for such a high quality. Set menus start at 900 THB. We took the one at 1200 THB and could barely finish because of the quantity... The food was simply excellent...
Around Chiang Mai: tour organized by Bungalows guesthouse (website is only in french) of the surroundings of Chiang Mai.
Elephant camp near Chiang Mai: we reserved through Bungalows guesthouse but in fact it was in collaboration with Panda Tour. Much better than the standard elephant show as you really get to learn how to climb on top of an elephant, guide it, and you ride an elephant bareback and bathe with them (incredibly nice and patient animals!).
Phanom Rung and Mueang Tam: tour organized by Old Phimai guesthouse. It is extremely convenient as these sites are a bit remote. The owner of the guesthouse was very nice and organized everything very carefully and nicely (after the tour we needed to be left at a different place to catch a bus, he organized all that very nicely).
Khao Yai National Park: Greenleaf guesthouse and tour organize tours in Khao Yai Park, looking for wildlife. It was very interesting, and these people are extremely nice and professional, they really know their way in the jungle and how to spot wildlife. The dinners at their guesthouse were also excellent.
Phang Nga Bay: