The National Arboretum is an oasis of trees and nature in the heart of downtown Washington. Tour the grounds by foot, bike or tram tour. And don't miss the National Bonsai & Penjing Museum (open daily 10-4) and the transplanted National Capitol Columns.
My family - ages 5 to grandparent visited the National Arboretum when the azaleas were in peak bloom. The hike through them was amazing. But even when the blooms aren't out, there's a fantastic bonsai display, a great children's garden (where kids can dig, water, and sometimes pick!), and the pond on Azalea Hill was loaded with chirping frogs the day we were there. For 2013 - the koi pond is closed! You can still sit on the veranda and enjoy a picnic, but the fish are no longer there while the pond gets rebuilt.
A few minutes' drive from downtown Washington, D.C. is a sprawling garden that pays homage to nature's bounty within the United States and around the world. Each season brings its own unique reasons to visit the Arboretum, fall colors and the cherry blossoms of the spring being the most attractive. The exhibit my kids enjoyed the most is their extensive bonsai collection. Room after room is filled with mind-bogglingly creative interpretations of trees.
Somehow the miniature sizes of the trees that they've seen only in massive sizes in real life seems to draw the kids. The thrill of seeing a pint-sized oak never seems to wane.
If you find yourself having to deal with a rainy day and you're all museumed-out, the bonsai exhibit at the National Arboretum is the place to go!