For campgrounds, I'll add another vote for Gettysburg CG. As far as what to do, that's a question that will elicit many differing answers. If you find history interesting then plan on spending as much time as you wish in the town of Gettysburg and the battlefields. We get to Gettysburg almost annually and we always find something or learn something new that we hadn't before. As others have suggested, start at the Visitor's Center and don't forget to take in the Cyclorama while there. You'll have many options for touring the battlefields; you can hire a guide, go on a packaged tour bus, take a self-directed tour by CD or even tour on horseback. You pick what you prefer and whatever your budget will support.
We also like the Adams County Winery off of Rt. 30 west of Gettysburg; there is also the Round Barn near Ortanna that offers fresh fruits & vegetables (in season) as well as locally-made jams, jellies, salad dressings and relishes. There are all kinds of dining available in town, from diner food (Dunlap's) to buffets (Gen. Pickett's) to fine dining (Cashtown Inn and others). There are several different "ghost tour" operators from which to choose. See also the Schriver House & the Jennie Wade house. There is also a tourist railroad in town. You'll find a vast array of bookstores, antique stores as well as the typical tourist-trap attractions (the Civil War Wax Museum is worth a look).
Enjoy & safe travels.