Wow, this thread is making me appreciate my parents for how many of these we got to. I grew up north of Chicago, so our quicker trips were all to WI, especially Devil's Lake and MN. I also loved Mackinac Island the the whole UP of Michigan.
Rocky Mountain National Park was our first venture west, and then of course we had to see Yellowstone and the Tetons.
As an adult I vote for Shenandoah (pretty much anything in VA) and Gettysburg National Park. We were lucky enough to do this one on a foggy day and you really *felt* what happened there.