If you haven't been to the Wellcome Collection – why not??
It's free and there are always wonderfully interesting exhibitions to see there.
On at the moment, until 17th June, is Brains– The Mind as Matter which is about brains. D'uh!
The Wellcome's permanent exhibition is also well worth looking in on; it features all sorts of fascinating and freaky surgical instruments and medically-related artefacts from all over the world.
Find the museum on the Euston Road, almost opposite Euston Station.