Wellow long barrow, the hill is not too big and there is an ice cream hut by the tiny car park. Take a torch for exploring.
Greenwich foot tunnel, lifts still being restored but it echoes and leaks a bit so you know you are under a big river. When you get to Greenwich park you can have a picnic on the meridian
Cerne Abbas giant, because it is a big willy
Lacock if your kids are into Harry Potter