If you are going as far as Stonehenge, see if you can get to Avebury as well. They have different tall rock formations and (as of last visit) you can walk directly among the stones. It's a great addition to the Stonehenge experience and is the same general area.

The other thing to do in either Bath or if you're outside of central London is get a recommendation for a good pub for lunch (preferably on a Sunday). The good ones have excellent food at reasonable prices and give you a great flavor for the English lifestyle. My favorite is in Wimbledon off the green called The Fox and Grapes. You can get there on the district tube line and take a cab from the station, or it's about a 1-1/2 mile decent walk if the weather's good.

Cheers, Anne