London's gallery of Modern and Contemporary Art is a real treat. Tate Modern is housed in a converted power station, and the building is worth a visit in its own right. Works on show may challenge, surprise, amuse or enchant you. The mix of well-known artists and more recent names on the art scene is fascinating, and the organisation of the gallery into four themed suites makes for a very different experience. You can take a break in one of the cafés and restaurants, all of which overlook the Thames with superb views of St Paul's Cathedral, or relax in one of the comfortable rest zones where you can browse books or listen to audio guides on themes relating to the art around you.
Entrance to Tate Modern and its sister gallery, Tate Britain, is free and there is a riverboat service to take you between the two.