The American Museum of Natural History
Okay, so I realize this isn’t exactly a ~hidden gem~ but it certainly doesn’t get as much credit as the Met or MoMa.
The American Museum of Natural History couldn't be more convenient to get to, just off the 81st Street subway stop. Check out the famous T-Rex skeleton, the giant blue whale, their new color exhibit, or just wander around outside in the hopes of having a Jerry Seinfeld sighting. He lives just across the street in The Beresford.
[Photo via @amnh]