You should spend at least one or two days in Bangkok if you have never been there before. The Royal Palace is fantastic. The various Wats are excellent, including several on the river that you can cross for about 1 cent US. The canal ride is fabulous, including stopping at the Royal Boat museum. There are other palaces and museums to see. The nightlife is really something if you are into that.

If you like beaches, you can fly cheaply to Samui or similar places.

Highland cities like Mae Hong Son, Chaing Rai or Chaing Mai are worth it.

A 1.5 hour train ride north of the Bangkok airport is Ayuthaya, and it along with Sukhotai farther north are wonderul places to visit. They are miniature Anghkor Wat like areas but more spread out.