If you like architecture this is a treat from Fosters & Partners engineered by Arup.
You can visit some of it Mondays to Thursdays from 8.30am to 6pm, Fridays from 8.30am to 5.30pm. One cool thing to see is the London Photomat (a picture of the whole of London you can walk on and explore) and the art exhibitions.
Two days a year you can see the whole building (check website for dates). Otherwise, weather permitting, you can have a sandwich at the Scoop (a nice amphitheatre outside)