Dulwich Picture Gallery is one of London's great smaller art museums, with a strong collection of old masters including Rembrandt, Rubens, Poussin, Murillo, Van Dyck, Watteau, and Gainsborough. The collection was assembled for the King of Poland in the 1790s and later found its home in Dulwich.
It is a good choice when the central museums feel too large or crowded. The gallery is compact, elegant, and easy to combine with a south London walk.