Dulwich Picture Gallery

Lending weight to the idea that the best things come in small packages, this (relatively) bijou building is the oldest public art gallery in the UK and its bright spaces house a brilliant collection  including Rembrandt, Rubens, Poussin and Gainsborough. It has recently branched out with an annual pavilion commission, just to give the Serpentine a run for its money, and its temporary shows are more than worth a trip across town for.