The city of Baltimore is only a 45 minute drive from Harrisburg and is pretty much a straight shot down interstate 83 south.
Here you can access Camden Yards for the lousy Orioles (coming from a Yankee fan), Inner Harbor is always worth a visit wether you like it or not but there's no denying that Baltimore's aquarium is one of the best, definitely worth a visit.
All else fails they have a 3 story barnes and noble to kill sometime in, and once you've found your book you can order it from your local privately owned bookstore at home.
"The Heart of Pennsylvania"
Not exactly a big tourist draw, Harrisburg. But it's the ideal base for touring central Pennsylvania. Moreover, the town has the most beautiful state capitol of any that I've seen (and you can tell that I've seen a lot of them).
It's also a pretty nice, laid-back city. The Susquehanna River is very attractive, and offers some great photo-ops. And I happened to visit during Labor Day weekend, when the Kipona Festival is on.
What makes Harrisburg such a great place for exploring the state? Hershey Park, Gettysburg, Strasburg, the Amish country, and many other attractions are close by. You haven't really seen Pennsylvania until you've been here.