It's a kewl area to take your parents to
As far as Stonehenge goes, if you are going to stay in Salisbury, there is a tour coach that departs from the coach station that goes to Stonehenge. I think the last coach departs Stonehenge at 3 in the afternoon. May want to check that for sure.
In Salisbury it's self, you should go to the Cathedral, and see the Magna Carta. Also, it's Market Day on Tuesdays and Saturdays, which is good fun. I go there alot myself. The atmosphere is really neat.
Bath is loads of fun as well. I recommend taking one of the tour buses around Bath, and that will give you an idea of what there is to see in the city. It's pretty easy to walk to most things. The Roman Baths are astounding, and alot more extensive than I had thought. If you have the time, see about taking a canal boat out of Bath for a day or two. It's a great way to see some of Georgian Bath, off the beaten track.
Lisa and Graham know lots more about the area than I do, so hopefully they will be along to tell you what 'not to miss'
Have fun