Depends on the type of vacation you are looking at- you can visit popular and populous Pattaya for water sports. Can go to Ayutthaya if you like History; visit temples, museums in Bangkok. Can visit Krabi for scenic beauty
November to April is good to visit Thailand.
Many Buddhist temples like Temple of reclining Buddha are there in Bangkok.Train passing through Thai market and Floating markets are also some other attractions. Pattaya is famous for its Coral Island and Walking Street. You can catch a night train to Chiag Mai and also visit Chiang Rai,a mountainous area 190 Km away from Chiang Mai.
Phuket Island is another attraction.Spend at least two days there.
The people are very tourist friendly. Thai Bath is their currency.[ 1Bath=2.10 INR]