The Gettysburg Battlefield(s) cover a huge amount of area (Wikipedia has it at nearly 4000 acres, or over 6 square miles. So, one great way to get an good sense of the lay of the land is the bus tour.
The doubledecker bus is a great option, and the view from the top is really good, better than from inside. You will, however, want to take weather/temperature into account if you choose to ride on top.
We made two stops, one for coffee/restroom, one at Little Round Top.
Our guide was very good, providing a narrated trip. They supply plug-in earbuds, like you can get on an airline flight.