Gettysburg is an incredible combined historical site. Of all the historic battlefields I have visited I would put them in this order:
Verdun: the starkness of it. There are 100,000 skulls of unidentified dead in the Douaumont Ossuary
Gettysburg: History's hinge is at Little Round Top. 24th Michigan was there.
Normandy: This one sprawls over an entire region but so many incredibly well-preserved sites. Omaha Beach, Bayeux, the memorial cemeteries, the mulberry, the Orne river and Caen canal bridges.
The Market Garden sites in Holland. The British Airborne Museum is definitely worth a visit.
I would still like to visit the Canadian WW1 battlefields in Belgium. if I had a lot of money i'd go to the Pacific battlefields (e.g., Bataan/Corregidor, Guadalcanal, Okinawa). I don't think you are allowed to go to Iwo Jima and Midway for different reasons. I might try to go to some of the Napoleonic battlefields in Spain because 1. its Spain and 2. I hear they are visually interesting.
I didn't think too much of Waterloo. Too much fawning over Napoleon and not over the Brits and Prussians who beat him.
There are a lot of battlefields in Scotland and the UK to visit but none of them seem particularly compelling like the above.