Of course Stonehenge must have its own giftshop. Actually, it's not that bad. I bought some stuff there myself. Although it carries the usual assortment of postcards, pencils with little stonehenge erasers, and placemats (?) it also has some really neat items as well that you probably won't find anywhere else. I got a great poster of Stonehenge at sunrise for very little. They had a good selection of history books on the subject, too, which were helpful. You probably won't find this stuff anywhere else, except maybe on the Internet, so I would advise you to buy while you're there if you see something that catches your fancy.
all sorts of stuff is available here from pencils with Roman era helmets at the top to erasers shaped like one of the stones. paperweights, candlestands, you name it. interesting and cheap gifts for people back home.