We've talked abt this with choco B who's going there
soon....Siam square, Mah Boon Krong, weekend Chatuchak mkt, and
some night mkt somewhere.
If you have more time, can go west to Kanchanaburi and spend a
couple of days (Bridge on River Kwai, JEATH railway/museum which
are WWII relics), or visit the King's summer palace or something
(forgot the name) which is a really nice park like 1-2 hrs from BKK
(it's not the other palace that looks like temple).
Best of all, remember to eat the authentic Thai food and snacks.
Have fun! 
