The United States Botanical Garden is a fantastic set of indoor gardens. The enormous greenhouse space features gardens, flowers, and plants from around the world, and is right off the National Mall.
If you love flowers and plants, you can easily spend a long time at the Botanical Garden, and the warm interior is a nice break from the cold, windy winters in the city.
Being on the National Mall, it is also a good option to throw in to a gap in your schedule if you are spending the day doing Smithsonian museums. However, it is definitely large enough to be worth a trip in and of itself.