Ayutthaya is about 1.5 hours away from Bangkok and you can hire a taxi for about $50 for the day (maybe you can get it cheaper but I was more than happy to pay $50 CAD for a day of air conditioned travel). It was one of my favorite day trips I have had.
I also really liked chatchuchak market but you have to go on a Saturday or Sunday. Also get there early to avoid the craziness.
If you are looking for a hotel I recommend Furama Asoke.
