I got the whole family to drive out with me to Rock Creek to see both of these sites. Peirce Mill (that's spelled correctly) was built in the 1820s and it's fun to poke around. There are NPS people there to answer your questions and from April-October they do live demonstrations of the mill two Saturdays a month. We'd actually seen this when we lived here before, but I didn't really know much about it then.
We also went over to Rock Creek Cemetery. It was established before the District of Colombia was and became a public city cemetery in the 1830s. It's large and interesting with a wide variety of stones. The oldest stones we saw were from the early 1800s but we didn't look for very long.
You can go by Metro to both of these but I think it's worth driving so you can get a better sense of how big and empty DC was in its early years. Peirce Mill is a bit of a walk from the Cleveland Park or Van Ness stops. Rock Creek Cemetery is nearish the Fort Totten Metro, but you have to walk down to its south end because the gate on North Capitol probably won't be open. It says it should be on Sundays, but it wasn't this past Sunday.