To get a feel for the size and lay of the land in Bangkok, get on the elevated train and ride from one end to the other and back.  It's a cheap, clean and safe way to get around, and the elevated view gives you a nice perspective.

     Hire a long-tail boat for a few hours to take you along the busy river and off into the klongs of the Thomburi side of the city.  Don't go with a group - just hire the boat and boatman for the two of you so you can call the shots.  It's a great way to see non-urban Thai life without leaving Bangkok.  The boatmen have spots where they stop and where they can get a commission if you buy something, but they aren't insistent - if you tell him no, he'll keep going.  The less time you waste at tourist stops or temples, the more time you'll have for getting way, way back in the narrow klongs.  Keep an eye out for a Washington license plate adorning the exterior of one rickety little wood-frame house on a backwater kong.

     Try to get out at dawn to see the sunrise reflecting off the mosaic-like exterior of temples - very cool as they glitter and glimmer.  The Thai temples are really nice - ornate - but for me, see a couple of them and you've seen enough. 

    Oh, if you're in Bangkok on Saturday, go on up to the Chatuchak Saturday market - it's a zoo, literally and figuratively.