The National Botanical Garden is a part of the Smithsonian Museums, which means its completely free to get it and features all kinds of plants and flowers from all over the world. There are so many different rooms and each one features different kinds of plants and flowers. There is a rain forest room, a desert room, Mediterranean room and countless others filled with beautiful plants. There is also an expansive outside garden with benches that you can hang out in and take in the nice weather. It is a great place to go if your looking for something to do near the Capitol building, or just looking for some cool Instagram pictures. It is open daily from 10am-5pm.
For more info check out: https://www.usbg.gov/