hi lucky
i just wrote a long answer to your post and pressed the wrong key on my laptop and it was gone. frustating...
so here for the short version: you have 3 weeks and a good budget to visit thailand, which is great.
some suggestions for cultural sights - bangkok, chiang mai, mae hong son, sukothai, ayuthaya.
i would also include a beach / island destination. not because you will find much of cultural interest there, but because thailand has the most beautiful beaches in the world (in my opinion).
in most places in thailand, you can visit bars and party. if you dont, you will be just as happy and also enjoy your trip.