Well I'd go for Hampton Court, an absolute must-see if you're keen on Tudor& Stuart history and well worth a day trip from London (take the boat one way if you do go). They have lots of people wandering around dressed in period costume playing various characters from the palace's history, e.g. Barbara Villiers. However, the palace gardens are being used for one of the cycling events for the Olympics so there might be closures and restrictions inn late July/August.

If you do choose Stonehenge then consider combining the trip with Bath, which isn't too far away, or, if stone circles & neolithic history really interest you then Avebury is within reasonable reach, too, with lots more stones, Silbury hill and the West Kennet Long Barrow, too. Definitely plan to see the stones as early in the day as you can, though, 'cos it gets very busy and your view won't be as good later.