Normal touristy stuff. Big Ben, Parliament, Buckingham Palace, rode the London Eye.
Went in the Churchill war rooms(pretty cool if you like history), Westminster Abbey, and Kensington Palace (wouldn't waste time going in, but the parks around it are really nice.)
Notting Hill Market actually has some pretty cool stuff. A lot of knock off antiques though. So if you want to buy anything major do your research.
My wife and I did a ton of traveling the 3 years we were in Europe. We found one of our favorite things to do when traveling to a big, historic city was to set a day aside to have no schedule or plan and just roam around an see what we find. You end up seeing a lot you might not otherwise. You also get a better sense of the actual city and culture that way. Wonder into random pubs off the beaten path too.