If you visit Plymouth, you can see THE Plymouth Rock. While smaller than most people think, it is a part of history, and I think everyone should visit it at least once. It's free to see, right on the waterfront, and about a 2 minute walk from the Mayflower II.

I don't think Stonehenge requires any sort of introduction, but it is an amazing place to visit, and one of the seven greatest wonders of the Medieval world.