I never knew some places were national parks, I swore the only national park I'd been to was The Grand Canyon, and I'd driven around Acadia. So, with no further ado, the national parks (and recreation areas, and monuments, whatever is listed at the national parks listing) I've visited.
Cape Cod National Seashore
Fire Island National Seashore
Golden Gate Recreation Area/Alcatraz/Presidio of San Francisco/San Francisco Maritime
Lake Mead (at least Hoover Dam)
Minute Man National Historical Park
National Mall/Washington Monument
Saguaro National Park (I believe... it's been awhile)
Statue of Liberty
I want to go to Devils Postpile when I'm next in Mammoth. Only problem, it's only open from mid June to mid October.