Under utilized DC spots:

The National Arboretum which is different than the botanical garden.

Jazz in the sculpture garden of the National Art museum in the summer. Free jazz but they sell food and some alcohol, you can just bring a blanket and food for a picnic, hide the booze at first other you’re good there too.

MLK memorial, closer to Jefferson and the Ellipse. FDR.

Malcom X aka Meridian Hill Park is bustling on a Sunday, usually includes a drum circle or sledding down the stairs on snow days. Otherwise don’t recommend it in the late evening.

Speakeasy’s. There are several throughout the city. The Shepard may have moved, it was previously very close to Dupont Circle. My favorite is in Eastern Market. You’ll likely have to go in and put your name on the list, then they text you when there’s room available. Then tell the bartender what you like and enjoy their work. Best found through some googling and Yelping.

National Cathedral, it has a denomination that escapes me at the moment, but they are famously wide open to other faiths. The gargoyle tour is fun but I recommend a tour of the main building. They’re nearly done repairing the spires post-earth quake so some tours are going up into restored areas. I recommend the basement too. Even the tapestries tell tales.