And, of course, there is the US Botanic Gardens. It's my favorite place on the National Mall, especially in winter when it is warm and tropical inside. But in spring and summer, the gardens are fabulous full of bulbs and colorful annual & perennial flowers. Each visit to the USBG you see tons of "tourists" (either actual tourists or locals enjoying the tourist attractions in the area) with their cameras out, intently photographing flowers and leaves and landscapes. For good reason, too: Not only are the flowers themselves very nice but in the conservatory there is excellent lighting. It's sunny but with shadows that hit the flowers in very, well, illuminating ways. You can really paint with light in there!
Here are some macro photos from my recent trip with, when possible, names listed:
Phragmipedium Grande (in color and black & white)
Another type of orchid
An unnamed lily
Note: Above photos are copyrighted material and should not be used without permission.