I don't know if she's going to Dorset, but: Corfe Castle (http://www.nationaltrust.org.uk/main/w-corfecastle)!
Also: don't want to reinforce a stereotype, but if she's American warn her that England is actually quite big. (Funny story: a, taxi driver, friend of mine picked someone up at Heathrow and was asked to drive them to Dublin, the American tourist failed to realise that it would cost him several hundred pounds unitl it was too late).
EDIT: Eden Project! Eden Project! And while she's in Cornwall (Truro's the best bit, well I was born their, which makes it the best bit) to get 1 Pasty and 1 Saffron cake.