>> visit Wat Pra Kaew and The Grand Palace.
>> cruise along Chao Praya River.
>> try thai food here, the original recipe, it's 10000 times better than thai food in other countries. Recommended menu including: Gang Cure Waan[Green paste curry] Somtam[papaya salad] Tom Kaa Gai[soup of coconut milk] Yum nüer [spicy n sour beef salad]. They will serve all of these dishes with the real Jusmine rice--Kao Hom Mali--if you go to fine restaurants. If you'd tried thai dishes before, u'll feel the different. It's much sourer, hotter, spicier and tastier.
>> Go to Silom at night, wheter you are G or str8. They are amazing.
>> Outside BKK: Chiangmai--try eat Khan Tok,a traditional menu set. visit Pratart doi Suthep. Try Kap moo, Num prick nume, Num Prick Ong, Sai-ouar and Gang Hung-lé. Go shopping at Gard Luang--the central market.
Phuket--Visit Lam Promthep,the cape. Try southern dishes: Gang Luang[yellow soup], fresh seafood inc. king-sized oysters, lobsters and more. Eat Gang Tai Pla, curry soup southern style. Take a trip around the bay, inc. stops at Maya bay[where they shoot the beach,Kao Tapu[the leaning Mountain], etc...
I think I mentioned the main principles, I'llleave the rest to Lonely Planet...