When I go back I'm definitely doing an excursion to Stonehenge
Bath or somewhere.
Paris: Definitely go to Versailles if you have time.
the key is to just take it slow. Decide what site you want to see that morning or afternoon and just go hang out in that area. Check out the shops. Stop in a cafe. Relax, and don't worry if you don't see everything. You'll go back.
No on Stone Henge, yes on Bath, yes on Versailles (make sure you go when the fountains will be on), and definitely yes on taking it slow. If you're in college, sure, go balls to the wall trying to see as much as possible the first time there if you are not sure how long it will be before you go back. But otherwise you'll end up trading in an enjoyable vacation for the ability to check off a lot of 'experiences' that would not be too much different from spending some time on Google Earth and some museum web pages.