American Museum of National History

Visitors per year: Almost 5 million
The draw: A science museum with dinosaur fossils, exhibitions chronicling virtually all known animal and human history and, yes, some eye-popping IMAX movie action
Highlights: Visitors to the American Museum of Natural History won't want to miss the Hayden Planetarium—which is slated to reopen in November—T. rex and friends, and that whale you've seen in a thousand pictures (it's in the Milstein Hall of Ocean Life). The Butterfly Conservatory, which features hundreds of live butterflies in a heated environment that feels like an oasis in winter, is also popular. Check the official website for the rundown on current exhibitions.
Best time to beat the crowd: Late afternoons or early on Sunday mornings