The Wellcome Collection is a free museum and library, mostly dealing with medecine and social history. The exhibitions are unusual and engaging, and the permanent collection includes bizarre items such as medieval masks and human remains.
Not the most exciting place inside, but a lovely rooftop area which not all that many people seem to know about, 2 mins walk from Old St station. If it's packed then The Red Lion is just up the road.
Although off the beaten track from the large national galleries, 198 is no less an important part of London’s artistic heritage. Developed during the social unrest in the Brixton area in the 1980s, 198 has grown from a community arts space into an exciting contemporary visual arts body that is often the first place to see the next rising star of the London art scene.