If it's still there see the Cyclorama, a large in the round painting of the high point of Pickett's charge. Devil's Den, a rock formation that was fought over, and Little Round Top, where Chamberlain had his moment of glory are also worth seeing. You might also want to do some of the Pennyslvania Dutch stuff further east near Lancaster, too. Some people really like it, though I don't know much about the details.
Thanks. This was for a trip on April 14.