My trip to the Orkney Islands was wonderful. I spent one full day and parts of two. With my friend I visited the Ring of Brodgar, the Maeshowe tomb, and the settlement of Skara Brae. All were very interesting.
The ring is very large and can be walked around freely (unlike, say, Stonehenge). You can walk right up to the slabs. It's right off the road.
The tomb is awe-inspiring. The space is maybe 15'x15'x15'. It is pretty completely dark except for the light from the guide's torch (flashlight for us Americans). My friend and I were the only folks in the tomb with the guide, and it was all the better for it. Not only do you get a 5000-year old stone tomb that took a hundred thousand man hours to build, but you also get a collection of 11th century Viking graffiti, too. The tomb is in a grass mound a short way from the road; you must buy a ticket (I think it was 10 quid) at the converted farmhouse opposite.
And the settlement is the most beautiful. Set right by the ocean, it is a sprawling area of holes in the ground, surrounded by rocks, and with some treasures inside. You can actually see a 5000-year old "stone dresser", which is clearly a shelving unit built out of rock for the room's inhabitants. It's still standing. On a nice day this is a really wonderful place to be. Another 5 or 10 quid.