You could speak incessantly about the Art- from the British Museum that stores the world in its rooms, or the National Gallery that leaves your head spinning by the sheer volume of its collection, or the art spilling from walls, staring up at you from the road.
Should the rains make you want to rush indoors, step into the massive National Gallery and admire Van Gogh's Sunflowers. Continue straight ahead from Trafalgar Square and you arrive at 10 Downing Street. The security here is massive, however you can get in few cheeky pictures :)
The Museums: London houses some of the best museums of the world and most of them don’t have an entry fee. The National Gallery is home to more than 2000 works of art. The plush Tate Modern and Tate Britain are also jaw dropping. Once in London the Science Museum is also worth a dekko which has seven floors of educating and entertaining exhibits.