I just came back 2 weeks ago with 5 of my best friends to celebrate all of our 30th birthdays and us being friends since we were just kids. Spent 21 days there (some of the time was in vietnam as well though)
What we did - Bangkok, we stayed at several locations, usually 2-3 days most at a time but most of them were around Khao San rd where the night life is pretty good.
Took a shuttle van down to Pattaya for a few days which is a nice city with beaches. The ride was a few hours but well worth it.
Took a day trip up to Ayuthaya(the old capitol) to check out all the old ruins. Was only a couple bucks to enter and there's several places that rent out bicycles for a couple bucks for the day which is quite nice on a hot day. Ride around the city and get some fresh air and a nice breeze.
Floating market was a cool experience but wasn't really impressive or anything.
Took a boat ride to Koh Phi Phi for a few days which was awesome as there's no cars on that island and the beaches are phenomenal. There's a small beach that has a swing set hanging off a tree. took about a 15 minute walk down a trail but when we got there it was completely dead so we had the entire beach to ourselves.
the beach. How could you say no to that??
Flew up to Chang Mai for a few days since domestic flights are pretty cheap!
Look up a book from Lonely Planet called "Southeast Asia on a shoestring". It goes in grave detail of places to go, the best places to stay, restaurants worth checking out, local scams, etc etc. It basically makes sure you get the most bang for your buck and get a total experience at the same time. Even has suggested itineraries based on how many day's you'll be spending there.
(if you're going to take a cab, some words of advice - make sure you ask them to use the meter and not a flat rate as the meter will be a LOT cheaper 95% of the time)