I guess if you ask a dozen different people you will get a dozen different answers.
-- One of the unadvertised and under-visited museums in Gettysburg is the Military Museum, a long brick building as you come back to the main road from Culp's hill. The collection of lots and lots of Civil War, WWI, and WWII materials there makes the Visitor Center's collection look anemic by comparison. It is a private collection, I highly recommend it if you wish to research just about any type of original artifact(s) including weapons, uniforms, musical instruments, etc etc. They don't advertise it because it is not handicapped accessible.. I don't understand but that is what I have been told. There is an armored WWII vehicle out front.
-- Another under-visited set of items are the two Whitworth breach-loading cannons near the eternal light memorial. I don't think too many people even notice them.
-- If you are a Freemason, there is the Friend-to-Friend memorial... very touching.
-- The wax museum apparently has a rare Confederate cannon out front that few if any people have noticed how rare it is.
-- The wall where the Federals fought their first battle of the first day is a must.
-- Little Roundtop and devil's den are must-see. You really need to walk across from Confederate avenue across and through devil's den and look up at Little Roundtop to get an idea of the challenges facing the Confederates. Its a *long* walk through an open field in heavy fire, against a fortified position.
-- Stand in front of General Lee's monument and take a good look across the open field at little round top.... field glasses might be helpful to see that far. You have to wonder why he didn't listen to Longstreet while you are looking from that viewpoint.
-- If you like buffets then try General Pickett's Buffet.
-- Oh, and don't forget Dirty Billy's Hats near the town square. I have purchased four of his hats so far.
-- If you can't afford a life guide or tour, then get yourself a self-guided tour recording and follow the signs.
-- You might wish to get "The Gettysburg Companion: A Guide to the Most Famous Battle of the Civil War" by Mark Adkin and/or "The Complete Gettysburg Guide" by J. David Petruzzi
-- Almost forgot, there were only two places during the battle to get water if you were thirsty, Spangler's Spring and a well in town. That must have been a very hot and thirsty battle in the heat of July. Spangler's Spring is a good shaded place to sit, rest and think. Not too many people go there either.
Personally my acquaintances and myself preferred the old visitors center, lots more stuff on display to study their appearance and how they were made and used, and less politically correct and less plastic. But that is just our opinion(s), mileage may vary. ;-)