Been there about 15 times as a teacher on school field trips, and went to college about 30 minutes from Gettysburg.
First stop has to be the visitor center. It is less than ten years old and very well done. Be sure to see the 20-30 minute movie narrated by Morgan Freeman. The cyclorama in there is impressive as well. You could hire a tour guide there if you really wanted the in depth experience. All of them are experts on explaining all the details of the battle. Must visits if you're driving through on your own are Seminary Ridge, the Virginia monument, Little Round top offers a great view of the battlefield, Devils Den is a group of huge boulders. that are fun climb around on. The Pennsylvania monument is really impressive, and the angle, or Copse of Trees represents the high water mark of the Confederacy during Pickett's charge on July 3, 1863. You can definitely grab a free map at the visitors center that can show you where all these places are.
When you're ready for something great to eat, head to the Spring Hill Tavern in The Dobbins House Restaurant. Be sure to go to the underground tavern, not the restaurant. Delicious food served in a colonial style setting by barmaids dressed in colonial attire. Plenty to eat and drink. There's plenty of good souvenir shopping in in downtown Gettysburg as well.
Enjoy!