Definitely bring a map! Some can be found at http://www.racemichaux.com/modules.php?name=Downloads&d_op=viewdownload&cid=16, which shows race courses mapped out by the Gettysburg Bike shop, which holds a series of races there annually. I've spent hours lost in the woods, unexpectedly riding over 30 miles. There are powerline trails, singletrack, fire roads, tough climbs, sweet downhills, beautiful scenery (definitely ride by the Pine Run reservoir), and all sorts of flora & fauna. Spend a lot of time there, and you can find what you like.
Take Route 30 east from Chambersburg or west from Gettysburg. Take 233 north (Pine Grove Road) at a traffic light. After about 2 miles take a left onto Shippensburg Road. Drive up the hill and there will be a snowmobile trail lot on the left (about 1.5 miles). Park and hit the trail. I usually ride down the fire road, take a left onto the powerline trail, then hang a right onto a great downhill trail.
http://www.singletracks.com/item.php?c=1&i=1700