The National Museum of Natural History is one of the most visited museums in the world — it holds over 145 million specimens of plants, animals, fossils, rocks, meteorites, and cultural artefacts. Free admission, open daily. The building itself, a classical Beaux-Arts structure with a massive central rotunda, is as impressive as what's inside.
The gem and mineral collection, anchored by the Hope Diamond, is the most popular gallery. The dinosaur halls (the Sant Ocean Hall) are excellent and have been significantly updated in recent years. The hall of human origins is thoughtful and sobering — it contextualises human evolution with material evidence rather than just graphics. For families, the live insect zoo on the ground floor is a crowd-pleaser.