Tate Modern is London's largest modern art museum. It is Britain's second most popular museum, with over five million visitors annually.
The Tate Modern building is an old power station at a great location along the Thames, where you can get to St. Paul's Cathedral via the Millennium Bridge. Tate is a great place to visit even if you aren't particularly a modern art fan, as the building itself is worth the visit. Visitors of the museum can walk the floor of the 30m tall power station hall, which makes the area look like a modern art piece.
Tate Modern was expanded in 2016, and from its 10th-floor terrace, you can get to see the area from a new perspective.
The nearest underground stations are Mansion House on the northern side of the Thames (10 mins away), and London Bridge on the southern side (15 mins away). Another good option is the Thames Clipper, with a dock right outside the Globe theatre next to Tate Modern.