Visiting Washington, DC. Anything I need to book in advance?

Sorry I wasn’t able to respond before I left on my trip, but I was busy cleaning, packing, and taking care of other last-minute business. I’m in DC now and would be happy to report upon my impressions after I leave tomorrow. In the meantime, I’ll just briefly tantalize you all with a cryptic photo of my favourite attraction so far. Nobody in this thread bothered to mention it, which is unsurprisingly, since architecturally it’s pretty unremarkable, though it is very close to a bunch of very beautiful buildings that probably should have been mentioned. I’m surprised the site is so popular—I was there for about half an hour, on two visits separated by a couple hours, and I was unable to snap any photos that didn’t have tourists excitedly snapping selfies on it.