There are lots of Smithsonians. What interests you? American History, Natural Science, Air and Space, American Indian, various art museums...?
www.si.edu
Make sure you have comfortable shoes for walking.
There is a hop on/hop off tourist bus that starts at Arlington Cemetary and goes around the mall/monument area. It might go to Union Station, too.
But you'll want comfortable shoes, anyway.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles