Friday afternoon and evening in London - ideas?

[RainDancer]

I've got the chance to spend Friday afternoon and evening in London with the OH but can't decide what we should do. I'm 19 weeks pregnant, so our usual pastime of spending the whole afternoon in a cocktail bar watching the world go by just won't do it this time! Any suggestions for something a bit different to do? (we live near London so have done most of the usual sights)

What about an exhibition? There are loads to choose from if you look at www.artfund.org/whatson or just meander round a gallery like Tate Modern, or a smaller place like the wonderful Wallace Collection nestled behind Oxford St with a glass ceilinged cafe bar and more gold framed old masters than you can shake a stick at (including the Laughing Cavalier).