Lots of options. You can get crammed onto rush hour trains, navigate overcrowded sidewalks, almost get run over by cars, get accosted by transsexuals, pay almost as much as you would back home for alcohol in bars, or spend time in sanitized, boring malls.
Kidding aside, my favorite things to do in BKK are:
Wander around Siam Square and see a movie for cheap at the retro Lido and Scala theaters.
Take the boats on the Chao Praya and watch the sunset at one of the many parks or restuarants on the banks of the river.
Spend time at Lumpini park exercising, people watching, and eating.
Watch the free fights at MBK on Wednesdays.
Wander Sukhumvit and people watch. Khao Sarn Road works good for this too.
Get out to a more relaxed place. Like Pattaya