I'm sure he going to be thrilled!!!
He'll need it if you inflict the monolithic might of the Tate Modern on him.
I wouldn't give him prior notice. I think it'd be a sterling suprise for him that way.
I'd also ignore the Harry Potter tours, I've never seen the films so I'm completely unaware of how many locations were actually shot in London, but it'd be easy enough to locate them using Google and Google Earth (to give you an idea of their approximate location and appearance). Maybe take stills from the DVD, print them out and produce a little mystery tour guide for him... that might break up the grown up stuff that you and Mrs ATD will want to indulge yourself in. Just a thought.