Stonehenge - take it or leave it
I had always imagined my first trip to Stonehenge to be magical, and for the most part it was. Try to get there early enough to watch the sun rise. And if you have the money, pay for a private tour. This allows you to go into the stones and sit for about an hour by yourself. We were there during the off-season, so bribed the security guard to let us sit in the stones before the park opened. That was amazing. As to touring during operating hours, it was somewhat of a disappointment. Most of the stones aren't the original stones, you can hardly get near them and the constant buzz of traffic takes away from the natural history you expect. (Supposedly the British Government is constructing tunnels in that area to remove the roads from around the stone cirle.) I recommend taking a side trip from here and going to Woodhenge or Avesbury. These are much more accessible and larger.