I visited the Salisbury museum today and it really does have a great deal of prehistorical stuff: the newish Stonehenge gallery is good and presented in a child-friendly way (without being childish). Current admission for adults is four pounds, but a voucher from a leaflet from the Tourist Information place knocks it down to three pounds. You could spend all day in this place (and conveniently the ticket allows re-admission on day of purchase).
Something is not right. This message is just to keep things from messing up down the road