You can likely purchase a Nok Air ticket close to when you leave, probably even same day (but it is a bit risky). My experience has been Asians in general do not book flights much in advance and so finding a last minute flight is not usually that difficult. You can book Nok on-line or go to one of the hordes of travel agents around everywhere. you could also just head to the airport and see what you come up with. Nok usually has some really cheap flight specials on-line, though.
As for staying in BKK, I would just stay downtown somewhere. The airport hotels are rather away from the city. If you want something to do the afternoon before your flight, try going to Panthip Plaza. It is a huge electronics mall - the taxi drivers will know of it. The ground floor also has lots of booths selling pirated DVDs, video games etc. However, what makes it somewhat unique is that they also sell CD's full of MP3s for about $2 each. For example, you can get a CD with the entire red hot Chili Peppers catalog on it in MP3 format for $2. Its an interesting place to just browse around. There is a really cheap food court up a couple of floors as well and the food is decent.
Enjoy your trip!