Absolutely incredible views of the White House and the National Mall. Fun and interactive cocktails, although... they ain't cheap. If you head there during the day you probably don't need a reservation, but it's always good to check just in case.
Most people think of the National Air and Space Museum or the National Museum of Natural History when they hear Smithsonian, but the Smithsonian Institution is actually made up of 23 museums (16 easily walkable in the downtown area)! They are all free to access, so I have put together a guide which gives some of my favorite highlights or exhibition recommendations to hit up. You can very easily hop in and out of all the museums, and definitely hit up multiple in the same day - they are kind of all right next to each other!
Views, views, VIEWS. Always on the hunt for the best view in DC, but you really cannot beat the top of the Washington Monument. Run by the National Park Service, you can reserve your tickets in advance, or wait in line on the morning you want to head up to secure a couple. PS. Yes, it's free.
An indoor market that has an amazing lunch counter (that also serves breakfast on the weekend!) The omelets are enormous and the vibe is so much fun. On the weekends, the streets outside are closed to thru traffic and it turns into a huge farmer's market.
Probably one of our favorite places in DC, please just bypass Georgetown Cupcake because b&w is is THE bakery you don't want to miss. Grab one of their seasonal flavors, a dirty chai latte, and enjoy your new favorite treats right outside on the historic C&O Canal. You're welcome.