Hard question to answer without knowing what interests you. You're right in that there is so much to do, and see.

You might think about a week in, and around Bangkok. There are tour agencies everywhere to give you an idea of what to see on day trips outside the city.
Then you can head up North to Chaing Mai for a few days if you like, or South to the beaches, but which ones....hehe.

I've found in my travels that it's best to have some general ideas of things you might like to see, and do, but to just play it by ear after you get to BKK. There is no problem with booking hotels/flights the day before you want to go somewhere. Once you get there there is so much that isn't in the guide books, or talked about on the internet that might interest you. Also the current weather conditions in particular places can play a big part in whether your trip is enjoyable, or rained out if you prebook the whole trip.

Just some thoughts.

koolbreez

Koolimaging