You won't be dissapointed by Orkney. A couple of good chambered tombs are cuween hill (well sign posted and clearly marked on the OS) there's a bit of a steep climb and you have to literally crawl on your hand and knees to get in. There's a torch in a box by the entrance but take a spare in case it's been nicked otherwise you won't be able to see a thing. Another good one we did was Unstan, close to Stromness. The tomb of the eagles is accessible by road from the mainland although on a different island. Didn't find this out untill I had left so missed it and was gutted!

Have fun