The Museum of Childhood is an excellent museum in Bethnal Green, housed in a fantastic building - read more about it here.
They are currently holding an exhibition of Olympic posters from the past 100 years. It's a really interesting group of work, because the thing they are celebrating is consistent, but obviously the different locations and the different eras have vastly different styles. The classic Olympic poster for me is the Mexico 68 one - it was great to see it in the flesh. There is some fantastic design there, including posters designed by Lichtenstein and David Hockney. You walk round it in chronological order, which is really interesting, the only down side of which is that you are left with the design for the London (iondon?) 2012 games. Which looks a little sad after its iconic predecessors. Still, for anyone interested in design, it's great collection to see. Find out more here.