Although the Ridgeway doesn't actually pass through Avebury, surely a visit to the area is a must either at the start or at the end?
As Harestone says you can avoid walking along the A4 back to Avebury from the Sanctuary by going through the fields behind the hedge on the south side of the main road. It is not marked on the map but in the summer the route is quite clear but in March it may be quite waterlogged. However you can see a few of the avenue stones that lead back to Avebury, in the hedgerow that shadows the main road. However to get there you will need to cross the A4 at some point.
There are plenty of places to see in and around Avebury which any good book will tell you about it.
Lastly A R Cane (AKA Steve) has put together a great 3-part blog on here about his walk along the Ridgeway which is worth a read and his photo's are brilliant too: http://www.themodernantiquarian.com/user/8822/weblog/