an excellent museum of modern art. There are Picasso, Dali, Rothko, Matisse and many others here. A free observation deck and a bunch of everything. Plus two excellent shops with souvenirs and books.
The place is interesting and it is necessary to visit it - at least in order to make your personal impression of contemporary art. The exposition is very large and partially paid - prepare at least a day if you want to look thoughtfully (and listen, there are many different formats).
In general, the Museum of Modern Art in San Francisco is cooler.