if i only had two weeks in the country, i would visit bangkok, chiang mai and sukhothai. though the beaches of thailand are really beautiful, they're hardly of the
most spectacular variety, and there's mostly just westerners hanging about there - why go abroad to meet more westerners?? chiang mai is a beautiful on the beaten path location with the possibility for hiking shopping and/or partying... sukhothai was the first capital of thailand and full of lots of 14th century buddhist temples (make it a day trip) (it's beautiful)... and bangkok, well, it's bangkok - make sure your're there when you have both a saturday and a sunday in the city... three words:
CHATUCHAK WEEKEND MARKET!
above all make sure to take it easy and take it all in - this is your holiday.
i loved the country nearly to death, hopefully you will too.