Baltimore is only something like 40 miles from DC, and NYC is about 100 miles away from King of Prussia. Plus,you get much better transportation options between two large cities than you do between a little suburban town and a big city in another state.
Just be aware, also, that Gettysburg will be a very long day trip. It's about 150 miles from KoP, around a 2.5 hour drive. I think if you look in the immediate Philadelphia area, you'll find more than enough to do to fill 3 days. Just in center city alone, there's the Independence Hall/Constitution Center stuff, Reading Terminal Market, and boatloads of assorted historical things to do. The Philadelphia Zoo is world-class, and is about a half-hour from King of Prussia. So is the art museum, and the Franklin Institute. The Adventure Aquarium is in Camden, you can go to Penn's Landing and get a boat across the river.
If you're into theme parks, Dorney Park in Allentown is about an hour and a half away, and as a PP mentioned, there's Hershey, too. Peddler's Village is lovely on a nice day, with dozens of shops and great eateries, and is about an hour from King of Prussia.