Guided tour of the National Cathedral is a must IMO. The Library of Congress is beautiful, and you can get a card while you're there. I held sheet music handwritten by Louis Armstrong. Love the Arboretum.

Also, small thing, but whenever I take visitors to the mall, we stop by the echo chamber outside the Canadian embassy. Always good for a video and a laugh 🙂

If you've got kids, and even if you don't, the National Building Museum is an overlooked gem (though it's not free).

Well, just eliminate the aforementioned manmade stuff and do the rest. Ocean, bay, mountains. Even though the slogan was made up by a beer executive, before it was the hub of Federal Government this really was the land of pleasant living.