I have no idea how I managed to miss this post- sorry! I suppose you have already worked out what you are going to do by now but , I will throw in my tuppence worth!
Chatuchak market- even if you don't buy anything it is a fascinating place. They sell anything you can think of- and some things you wouldn't think of!
Wat Po- with the reclining Buddha
China town- take a walk around there. We actually haven't done that yet- we only did this as part of an organised trip. I plan to go off on my own this time though.
Jim Thomsons House- quite an oasis in amongst bedlam. Very interesting story there.
Kanchanaburi and the River Kwai bridge-this would be one of my all time favourite trips. You visit the Jeath War Museum. This is on the site of the prisoner of war camp. It is quite gruelling but a must. You visit the war cemetary - it is beautifully kept and I found it so moving. Read the inscriptions on the graves and it is impossible not to shed a few tears.
You go on to the actual bridge and can walk across it- ( note I didn't say original as it was completely destroyed once and then badly bombed again later on) . I have to say that the walk is not for the faint hearted. There just has to have accidents as it is pretty rickety and open. Ok until someone comes the other way and wants past! You can then the train itself on a journey through the countryside. I loved this as well. A long day but worth it.
A visit to Ayutthaya - the ancient capital.The Burmese all but destroyed it in the 18th century. You can see the ruins today. On the way you stop at the Kings Summer Palace.
There are other trips I could suggest. I haven't finished exploring Bangkok yet- ( can you ever I wonder!!)
Hope this is gives you a start anyway.