I had 3 hours to kill in London so I went to the National Portrait Gallery. It is centrally located in the city and easy to get to by walking. The bus system in London requires an Oyster card so plan on walking or taking the Metro unless you already have an oyster card on hand. The National portrait gallery blends history and art with portraits of notable British figures from the medieval era to modern times. and you can maximize your time by eating in the cafe. And it's free! In addition, you can grab a bite to eat in the cafe. It has sweets as well as more hearty salad and sandwich options (and a wine and beer selection!).
Great place to kill a couple of hours and it has a nice cafe attached to sit back and relax and grab a bite to eat. There is also a rooftop restaurant for a more substantial meal as well: http://www.npg.org.uk/visit/shop-eat-drink/.