This one is self-explanatory, but just in case... Head down to the National Mall to see the sites and visit the Smithsonian museums. Mostly free, these sites will not disappoint!
If you want to get out on the water, you can rent kayaks, stand-up paddleboards, and other items here. If you just want to sit by the water, there are tons of spots along the C&O canal to bring a picnic.