The United States Botanical Garden, I had a good time. They were having a special orchid exhibit, which were lovely to look at.
A little historical fact: "The institution actually traces its beginning to 1816, when the constitution of the Columbian Institute for the Promotion of Arts and Sciences in Washington, D.C., proposed the creation of a botanic garden to collect, grow, and distribute plants of this and other countries that might contribute to the welfare of the American people. "
The other areas of the gardens included different environments such as one for swamp flora and a desert.
As you can tell I mostly enjoyed photographing the plant life and using my new pancake lens. I'm sure I would have gotten a lot more, had there been more time. But I have always enjoyed the plant life from the desert and their many shapes and forms.
Overall a must see on your trip to D.C. as it is all simple pleasure and not loaded with politics or grand artistic value. Just beautiful flowers and interesting plant life, to help sooth the traveled soul.