Just moved out of DC after a decade. Here’s a combo of hacks, tips, and suggestions:

1) Metro can be MADDENING. Check for scheduled work and unexpected delays before embarking: https://www.wmata.com/service/status/index.cfm

2) Use the Circulator — $1 bus with maybe six routes. Beware that the routes all have different hours (and sometimes seasons) of operation. One route goes down the Mall, around the Tidal Basin, and back down the other side of the Mall. Possibly the best way to get to the monuments and many museums. 

3) For a great off-Mall museum, check out the National Building Museum. Worth the fairly low entrance fee. Exhibitions focus on architecture, building, planning, urban issues, etc. Usually really well done. Alternatively, entrance to the building alone (not the exhibitions) is free and the building itself is worth a visit.

4) Only two Smithsonian museums have good food — American Indian and NMAAHC. Not cheap, though. If you’re around the Mall at lunch, check out the food trucks at L’Enfant Plaza.

5) If you enjoy history and have time for some walking, check out Cultural Tourism DC’s self-guided walking tours. Good way to learn about the city’s heritage — not just as the nation’s capital.

6) If you want to go to the Kennedy Center but don’t have a car, it’s a reasonable walk from the GWU/Foggy Bottom Metro station or there’s a free shuttle!

7) Union Station is accessible from the rear — H Street NE — through the garage. It’s typically easier to get through the building through that entrance than the main entrance with all the taxis, tour buses, etc.

That’s what I’ve got at the moment. I’ll reply to this if I think of more.