What can I do/see in two to three day trip to Bangkok/Thailand?

I too think if you can anyway possibly fit it in, go to Chiangmai they have an awesome evening market. Make sure you have correct change ( not big bills) for taxis etc. as it is a not uncommon to pay with big bills and then end up with change being given back to you in counterfeit bills. Also make sure that you have your hotel name etc. written down for you in Thai on a card and carry it with you at all times. While many people speak English more don't, and it is terrifying to go out somewhere and then not be able to make yourself understood well enough to get back to your hotel. Best wishes for a great trip!