National Museum of Natural History

With around 7 million visitors a year this is the most visited natural history museum in the world and the second most visited museum in the United States. Opened in 1910, it holds over 126 million specimens and employs 185 research scientists, more than any other museum. The mammal collection is by far the largest in existence and the gem, mineral and meteorite collections rank among the most important anywhere.

Highlights: the Hope Diamond in the gem hall, the African elephant in the rotunda, the Hall of Fossils with a complete T. rex (one of only six places in the country where you can see one), the Ocean Hall and the live insect zoo. The building appears in Night at the Museum: Battle of the Smithsonian and Get Smart.

Practical: Free, no ticket needed. Entrances on Constitution Avenue and on the Mall side. Expect security screening and queues around opening time in summer. Two hours covers the highlights; the building is big enough to fill a day.