Yeah don't miss out on Avebury! I know Stonehenge is easy to get to and it's always full of people so as much as it's absolutely amazing, I didn't enjoy it as much as other sites where there are barely any tourists.
North of Scotland is incredible too... I was in Orkney island in Autumn and I was alone
in those stone circles... what I felt there I just can't explain. Just fantastic.The only downside of the islands is you have to walk a lot or have a car available. I was going on bike and it was quite rough with the crazy weather. xD
I didn't know about the Chaco culture until now, but hey that should be quite easier for you to get to.