The Tate Modern is an awesome sight even before you get to it you can see it looming in front of you like a giant russian electric company. The new'ish access foot bridge is also a gem and worth taking a good look at. The Tate can be quite crowded but with art going back 600 years its easy to understand why. Go early though. The walks around the Tate are pretty cool, the river and the banks are quite an little adventure.