The new Rubell Museum is free to DC residents and has awesome artwork (get a daily pass online in advance). You can walk to the Wharf after, along the water which can be a cute little outing.

You can go up inside the Washington Monument (with passes). See the National Museum of African American History and Culture too since it’s right there (with passes).

Renwick gallery by the White House is really cool too and has rotating exhibits. No timed passes needed.

Walk through Rock Creek Park. There are tons of trails and it feels like you’re not in the city.

National Arboretum is really cool especially when the flowers are in bloom.

Arlington National Cemetery if that’s your thing.

DCist has a weekly “Things to Do” in DC that usually lists the prices of upcoming events and stuff happening. Many are free.