The New Tate Modern has an amazing collection of Modern Art. If you could pick only one museum in the world to visit the Tate should be at the top of your list!
Aside from the art collection, the building itself has a 365 degree viewing platform on the 10th floor. Because visiting both the museum and the viewing platform on the same day might be too difficult a trip, I’ve created three books: one for the art museum alone, one for the viewing platform alone and one with both. This will give you some options if you need to segment your trip.