I keep forgetting to take the camera with me, but fortunately these places all have plenty of public domain photos out there.
Whenever we'd drive into Washington on Constitution Avenue, I'd always wonder what the little stone house on the corner of 17th Street was. That's not exactly a convenient place to stop to read the sign and I'd always forget to check when I got home, but now I know. It was a lockkeeper's house where the Washington branch of the C&O Canal joined the old Tiber Creek and the Washington City Canal.
We looked at some old maps and drawings before we went to help us visualize what this part of the city would have looked like around 1840 and the house has a map there too. We also walked along Constitution Ave which is where the Washington City Canal went for part of its distance (it went down into the Anacostia River near the Navy Yard). The lockkeeper's house is the only known visible remnant of the canal. You can, of course, still see the Washington Branch of the C&O Canal in Georgetown.
We also rode down to the Navy Yard and saw the barracks and Latrobe Gate. The gate has been completely altered, but it was one of the few public buildings the British didn't burn in 1814 (the Commandant's House near 8th and G is another one you can see). This is an important site for the War of 1812 in Washington DC, but you really can't see much without an escort (which is doable, apparently, but we didn't do it).
The Congressional Cemetery was another pleasant spot with a number of famous people buried there. William Thornton is a man we've read about a lot since he designed the US Capitol and then spent a lot of time arguing with people about actually getting it built since a lot of his plan wasn't actually structurally sound. This isn't a place you must go but I thought it was worth stopping at near the Navy Yard. If we could have gone to the Naval Museum and seen the display on the War of 1812, we probably would have skipped this.
Depending on which way you're coming from on the Metro, you can either get off at one of the Farragut stops, Federal Triangle, or the Smithsonian stop to get to the lockkeeper's house. They're all a little less than a mile away. The Barracks and Navy Yard can be accessed from the Navy Yard stop (how surprising) or Eastern Market if you're on the orange, blue, or silver line. We walked to the Congressional Cemetery from Latrobe's Gate, but you can get there from the Potomac Ave stop or the Station Armory stop.