Though it's below freezing in DC you can still enjoy a bit of summer. The National Zoo offers a free butterfly exhibit that is a delight. Leaving the cold outside you walk through a double set of doors into a wonderland of butterflies flying around and blooming flowers.
If you stand still you might even get a few of the butterflies to land on you. It's a nice way to warm up after wandering through the zoo and of course a place to take a few photos too.