Whoo hooo! A request about PA...an area I'm familiar with!! <img src="/forums/images/graemlins/tongue.gif" alt="" /> I'm not entirely certain how far you want to travel to hike but I can recommend some places.
Since you'll be in Waynesboro, the first thing that comes to my mind is South Mountain State Park and the Appalachian Trail. It's actually a Maryland park, but it's not far at all from where you're at...about a 15 minute drive. A good section of the AT runs right through it, too. Here's a link to the park's website:
http://www.dnr.state.md.us/publiclands/western/southmountain.html. You could easily spend all day there.
My second thought would require about a 60-90 minute drive north to Colonel Denning State Park. It's one of my favorite day hikes. There's a trail there that's called Flat Rock Trail. It's a 2.5 mile hike up the mountain (somewhat steep at places) but at the top is a beautiful view overlooking the Cumberland Valley. I normally make it up the trail in about 70-90 minutes depending on my pace. The good part is that it intersects with the Tuscarora Trail (a 105 mile trail) so there's more to hike than just the Flat Rock trail. You've also got many thousands of acres of state forest and state game lands right there, too. BTW, the homepage for this state park is
http://www.dcnr.state.pa.us/stateparks/parks/coloneldenning.aspx.
My third and final suggestion would be a place I've yet to visit, but I've always wanted to and will do so soon. It's Cowans Gap State Park and the Buchanan State Forest. It's about 60-90 minutes NW of Waynesboro. I can't comment on it much other than pictures I've seen have looked beautiful and I know there are trails there (but I'm not sure on length or anything). Here's the URL's to both:
http://www.dcnr.state.pa.us/StateParks/parks/cowansgap.aspx and
http://www.dcnr.state.pa.us/forestry/stateforests/buchanan.aspx.
I hope this helps! Feel free to let me know if you need more specifics or anything. I'll see what I can do.