My wife and I really enjoyed our visit to the Courtauld gallery. The impressionist collection in particular is well worth a visit with some major pieces by major artists (Degas, van Gogh, Cézanne, Manet, Monet, Morisot). It's also an ideal size, I think it took us maybe 2 hours to go around. The staff were friendly and helpful and the works are very accessible (not many are behind glass). There are free tours and the café is nice (if expensive). I would highly recommend a visit to this gallery. I would also mention that the shop have some lovely and thoughtful products to buy.