The National Arboretum is one of the hidden gems of Washington, DC. The Arboretum has guided tours during the day, or if you want to go by yourself, there are more than six miles of walking trails. If you don’t want to walk everywhere, you can drive through the Arobretum and park at one of the many parking areas.
The Arboretum has a huge collection of Bonsai trees–some are hundreds of years old! At the entrance there is a Koi pond, and further in there is a monument built from columns that were originally part of the Capitol Building. My favorite time to visit is during the spring when the Cherry Blossoms are in bloom. The Arboretum is a good alternative to the Tidal Basin because it isn’t overcrowded. Also there are many types of Cherry Blossoms at the Arboretum that bloom throughout the year, wheras the Cherry Blossoms at the Tidal Basin all bloom at one time.
In order to get to the Arboretum you will need to drive or take the bus, because it isn’t located on the National Mall. I recommend visiting several times in order to see Azaleas and the other flowers that bloom at different times during the year.