Maybe Avebury? seems fairly central. We could do a walk taking in the large circle:
Take Herepath up to the ridgeway from the village, then south to the Sanctuary.
Go along the A4 to West Kennet long barrow and Silbury hill,
then take the foot path across the fields towards the Beckhampton road and the village again.
It's a nice walk, about 5 miles, no steep hills, other than the one up to the W Kennet barrow, and at a slow ramble it will probably take a day.