Good you mention Maes Howe. I was there this year and loved it, despite the fact that because it was a guided tour I couldn't sit around, light a tea light, smoke a spliff and write a poem and compose a song and write a pagan screen-play.
I was hacked off with the metal gate there, otherwise I woulda been in in the middle of the night. (I wouldn't have gotten scared and ran out screaming, honest.)
Some folk manage to get it nearly to themselves, by judicious timing on the last tour of the day. I know what you mean though, it would be nice to have the option of a guide, but not mandatory.