Depending on your route to/from, I'd look into stopping at the National Museum of the Air Force in Dayton. We did this on the spur of the moment traveling through, and wished we would've planned more time for it. It was very interesting!
Since you like theme parks I'd make a point of going to Cedar Point if you've never been there. It's been quite some time since I've been there, but it's the best amusement park for coasters I've ever been to.
On your way to the Hershey area, you could stop at the Pennsylvania Grand Canyon. Of course it's nothing like the real Grand Canyon, but it's a really nice area. We aren't hikers, but since you like outdoor activities, I'm sure you could find some hiking trails. We like the overlooks and just driving through the area. When I was a kid, we went every fall to see the leaves changing colors.
In the Hershey area, you have Hershey Park for another theme park. Since you're into history, I'd make a point of going to the Civil War Museum in Harrisburg. We went a couple years ago and loved it. Even the non history loving people, but I really loved it being into American history. Id prioritize Cedar Point over Hershey park, and use the day around Hershey area to do the Civil War museum and maybe the Hershey Story where you can taste chocolate and I believe make your own. As mentioned Gettysburg too, but I've never been there.
Lancaster is a nice area. I don't know if you've been to the Sight and Sound theater in MO, but there is one in Lancaster and I hear it's really a great show. I've never been there, but it's on my bucket list. I would look into taking one of the buggy tours while you're there too. Some have tours where you stop on working farms, and they're pretty neat for something different. Stop by Kitchen Kettle Village while you're there too. It's a little shopping area. Also, go to one of the smorgasbords for dinner.
I've never been to Philly. Being from Pittsburgh and frequenting Atlantic City I've been through many, many times, but never visited.
While you're in Baltimore, the aquarium and Fort McHenry are must do's! The inner harbor is pretty. We used to stay right there, and enjoyed just walking/sitting around the harbor.
You'd be close to D.C. while in Baltimore, but that would take up a few days.
Can't really help on the way back through WV. Never done much visiting there. Harpers Ferry is more towards Baltimore in WV. Never been there either, but it's a historical area.
Of course you won't be able to do all this (well, you could if you had a couple weeks off), but it's some things you could look into. My favorites would be: Cedar Point, Civil War Museum, Fort McHenry, and the Air Force museum. I really like everything I mentioned, but these are the places I'd really make a point to see.
