A 163-acre reserve carved out of Rock Creek Park, the Smithsonian's zoological arm is a long-standing family favorite, where creatures from aardvarks to zebras go about their business in what are mostly generous outdoor enclosures. It's predictably popular on warm weekend days, and Washington's celebrity panda cub, Tai Shan, in particular, can draw oppressive crowds. Keep walking and you'll find some quiet corners and genuine wildlife watching: golden lion tamarins (tiny Brazilian primates) live unrestricted in trees along one of the zoo's paths, and watching them jump from branch to branch with no fences between you is an unaccustomed thrill. The $52 million Asia Trail, whose bridges and paths wind by quirky otters and lumbering sloth bears, opened in 2007.
Tip: The zoo grounds open at 6 a.m., the buildings at 10, leaving four hours when early risers—human and animal—have the place to themselves.