Archive collection of surgical instruments - anatomy and pathology - dental instruments - medicine and surgery (basically a lot of dead things in jars, really interesting to draw from).
Travelling on these uncrowded river buses is such a relaxing way to see the city's landmarks from the Thames. A really quick and lesser-known way to travel from central London to Greenwich, especially on a sunny weekend.
On a hot day there is nothing better to do in London then go for a swim in the Heath ponds. You get completely transported you out of the city as Hampstead feels like being in the countryside except you're in central London?!
If you want a peaceful day out of the busy city, Highgate doesn't really feel like London. The cemetery is interesting and peaceful, there's a tour of the old half which is definitely worth the money. Karl Marx, among others are buried there.