Order the driving tour, can't remember where I Ordered it from but it makes the tour of Gettysburg better. Also watch the movie Gettysburg before you go showed it to my wife our first night there and she gave me heck that I hadn't made her watch it before that. It made you feel like you could see the soldiers fighting there when you got to the battle field.
Gettysburg is one of those places that you need to go back a second time to take it all in.
If you go to the chamber of commerce site they will send you a packet of all the things to see.
My wife's favorite spot was Devil's Den
I think 4th of July they usually have a reenactment of parts of the battle.
This is the Driving tour I bought