Mentioned by Santorini Dave
Washington, DC with Kids - 26 Best Tours & Things to Do
"Just down the hill from the Capitol building is a place where children can stretch their legs and everyone can enjoy a diverse oasis of green. Sparked by George Washington’s notion that a national garden would demonstrate the importance of botany, the United States Botanic Garden was established by Congress in 1820 and has since grown into an impressive showplace, a museum of plants from the desert to the tropics. The glass and stone Conservatory offers a canopy walk above a tropical rainforest and a collection of 5000 orchid varieties."