It seems almost unimaginable that only five years ago I went to Orkney in the middle of March and had The Ring Of Brodgar to myself for an hour in the morning, ditto Stenness for half an hour; there were only four of us visiting Maes Howe and no-one else at Skara Brae. I went back to Brodgar in the evening and spent another solitary half hour or so. These places are increasingly on the tourist trail and tjj is right; if you want to avoid the hordes you've got to go out of season and take your chances with the weather. I'm currently planning a return to Callanish in mid-October which I reckon should be ok for not having to share the monuments with dozens of others though the last time I went at that time the wind was so strong the return ferry crossing had to be cancelled.