In my opinion, the Library of Congress is the most beautiful public interior space in DC. I only recently discovered this treasure which is shocking considering I've lived in Virginia for twenty years! Not only is it visually stunning but it's the largest library in the world, and has an incredibly interesting history. There are several excellent exhibits including, Exploring the Early Americas, which an absolutely fascinating look at the times of Columbus and other explorers "discovering" the Americas.
There are so many things to tout I won't even begin to try. Last weekend, I went for the third time and plan to go again and again. If you haven't been, it's a must see!
Photography info: Images were reshaped using the Enlight app.