+1 for Avebury. We visited it on the winter solstice a few years ago, and had no issues whatsoever with crowds. You can literally walk up and touch the stones and walk in the ditches and mounds (unlike Stonehenge).
(The Visitor Centre did a fantastic Port & Stilton Soup the day we visited - which was EXACTLY what you wanted for lunch after walking around in the sleet for a couple of hours!)
There are lots of neolithic sites worth visiting in the area
https://en.wikipedia.org/wiki/Stoneh...sociated_Sites - in particular, don't miss Silbury Hill
https://en.wikipedia.org/wiki/Silbury_Hill and take in a couple of the barrows (tombs) such as West Kennet Long Barrow
https://en.wikipedia.org/wiki/West_Kennet_Long_Barrow