I have some American friends over at the moment (ferrying them out to Heathrow this evening) but if you're bored on Sunday and want to meet up for a beer just post here and I'm sure you'll find some takers.
Yep, I'm in town on Sunday with my wife so I'm happy to meet up for a nice glass of wine or two.
The Tate modern http://www.tate.org.uk/modern/ on the south bank across the footbridge from St Paul's is worth a look if you like modern art, that's free too.
A cool Geeky place to go is the Science Museum
http://www.sciencemuseum.org.uk/ in Kensington which is one of the best in the world. With the National History museum http://www.nhm.ac.uk/ next door you'll easily have a good time and no doubt learn a few things ...
and it's all free!
The first thing to do is the London Eye http://www.londoneye.com/. If anything it gives you your bearings in a beautiful and fun way.