Visiting Sukhothai is an experience that I wouldn’t want to have missed:
http://www.43places.com/places/view/937858/sukhothai-sukhothai-province-thailand

VIsiting Chiang Mai was very nice as well. The city center is so small you can easily walk around it:
http://www.43places.com/places/view/105069/chiang-mai-thailand

Staying in Chiang Mai was excellent at the affordable Manathai Village:
http://www.43places.com/places/view/929872/manathai-village-chiang-mai-thailand

Have a great trip!

Don’t miss the Loi Krathong festival, and esp. in Chiang Mai

If you’re adventorous, and have a strong stomach, there are lots of dishes to try. Side corner restaurants and street vendors have the most authentic stuff. Kaow Niaow and Sum Tum (Sticky rice and spicy Papaya salad) is good. Kaow Tum is a kind of rice porridge eaten with lots of side dishes, vegetables, chinese sausage, salted fish, salted egg, etc. Yum Pla Duk Phuu is a catfish dish I always liked.

I’d echo the comment about Sukothai above, a similar place is Ayuthhaya. For Beachs Koh Chang (Elephant Island) was the more remote and relaxing, Koh Samuai has nice beaches, but more people and night life.

I also really enjoyed my experience in Chiang Mai. One of the highlights for me was attending a day course at the Chaing Mai Cooking School. If you’re interested in Thai cuisine, you’ll enjoy it: http://www.thaicookeryschool.com/

We stayed in Chiang Mai at the D2 Hotel – a chic boutique hotel that we really enjoyed. http://www.dusit.com/hotels/thailand/chiang_mai/dusitd2/index.html

Also, this is a wonderful site to check out before your trip about Thai ingredients, culture and cooking, run by an American expat in Bangkok. http://www.realthairecipes.com/

While you’re in Chiang Mai, check out the little town of Pai. The food all over Thailand was great, but in Pai, I found the most densely packed array of yummy cuisines from all over the world thanks to the many foreigners who have set up shop in Pai. There are also waterfalls and hotsprings not far away. It’s just one of those places that inspires contented lazy days, no need to rush nowhere. Mai bpen rai…

Go to the MBK for the utter sparkliness of it. It’s a shopping mall with a zillion stores. Lots of shiny things. lots of tiny little vendors. There are even some brand-name stores you might recognize sprinkled in.

If you’re into modern consumerism this is the place to go.