As earlier post mentioned, check the BKK and TG forums for lots of ideas.

Your schedule really does not allow much time for a hotel, so you might want to consider just hanging out in the terminal (ugh!). However, if you really want a hotel, there is the very convenient but pricey Amari (I paid about US $135 last summer) and the Miracle Grand, which is quite nice and substantially cheaper at about US $55. Check out Asia Hotels.com. (Apologies for the plug, but they have worked well for me.)