I spent half of June in Delaware for a class & while I was there I drove down to the Capital for some good old fashioned sight-seeing. I hadn’t been to DC since junior high (except when I was in my college band and marched in the 1997 inaugural parade for President Clinton). I spent almost a full day in the National Mall and another day at Arlington Cemetery. A short walk from there is Marine Corp Memorial (Iwo Jima statue).
By the way, if you’re in the area, I highly recommend the DC by Foot walking tour. It’s free but your tour guide will surely earn a gracious tip.