We live in London, so he's done most of the obvious ones- Natural History, Science Museum, Transport Museum, Aqaurium, Zoo and I don't think DH and I could stomach soft play, Eddie Catz type place so somewhere interesting for him but not too awful for us. There must be hidden gems that we haven't heard of/thought of. He's into normal three year old stuff. He loved all of the above, though wasn't greatly interested in zoo. He's also had a boat trip on Thames which he wasn't that fussed by. We're willing to drive out a bit, and I thought of Legoland but it's shut tomorrow.