Well, more than that, London has some of the world's best museums. With the Tate Modern housing some amazing modern art, the V&A housing everything under the sun, the British Museum with all of the stolen artifacts (I mean recovered artifacts) the archaeologists could carry, and the Strand being some of of the city's museum highlights.
As with all museums, these are also great options for leaving a bag for the day. Most coat rooms will accept carry-on sized bags. Generally fees are quite low and well worth the convenience of not having to tote around a large bag as you site see. You can check each museums website for more detailed information.
THE BRITISH MUSEUM
The British Musuem houses amazing insite into the ancient world, namely their amazing mummy collection. It's easy to debate whether or not these mummies should be in England in the first place, but while they are the British Museum provides an excellent venue for peeking into history. I wish I had planned for more time at the museum as an hour was certainly not enough--we practically ran through the museum.
Getting to the museum is relatively simple, but requires some walking. Directions are available at the tube stops. The closest three stops are: Tottenham Court Road and Holborn, on the Central line; and Russel Square, on the Picadily line.
The Tate has always been one of my favorite museums in the world. The unassuming building gives no illusion to the extraordinary collection of modern art inside. This particular trip, I spent much of my time in the Rothko room, but I have never been disappointed by any of the art housed at this museum.
There are a few underground and public transit stops around the Tate. You can use Blackfriars and cross the bridge, take boat to Bankside Pier, or the Overground to London Waterloo and Southwark stations. You can also get off at Mansion House Station and walk across the Millennium Bridge--a recommended walk no matter where you decide to stop.
VICTORIA AND ALBERT MUSEUM
Plan to spend at least half a day if not an entire day exploring the V&A. The place is filled with a variety of objects, from metal working to ancient armor to modern fashion--something for everyone. If you've been before, you probably haven't seen it all. But, if you've been many times, there are always temporary exhibits that might tickle your fancy.
The V&A is also a nice place to stop and get lunch. The museum hosts a large cafeteria style area with pretty good food at relatively acceptable prices (about the same as going out to a pub in the street, if not just a bit cheaper).
Getting to the V&A is easy. You can take the District line or Circle line to South Kensington Underground Station. From there you will see plenty of signs directing you to the museum and also the Natural History Museum and Science Museum. I've yet to be to either, but I'm sure they're awesome!