One of my favourite Avebury walks, js, starts at the bottom of the Kennet Avenue (the end away from Avebury) and up Waden Hill. At the top there's a spectacular view of Silbury (pic here http://photos1.blogger.com/blogger/6094/1623/400/Silbury%2024.0.jpg ). You can then walk down the other side of Waden, past Silbury and on up to West Kennet Long Barrow (which will also take you past Swallowhead) or retrace your steps to the Avenue. If you decide to do the whole walk you don't have to climb Waden again, just follow the path along the Winterbourne and that will bring you back to the main Avebury car-park.

Sigh, wish I were there right now. Enjoy :-)