The
scooter idea is a good one. You'll be able to see more and not get as tired using one. The average walking in a park is about 6 miles. Many have done the pedometer to verify these results. They can always park the scooters if you want to walk some.
I'm older and my hubby is 77 and we've gone with older friends who are healthy and not as healthy and you need to pace yourself. Alternate activities so there will be rest periods. For example, do a ride and then a show, do a ride and then a snack, do another ride and then another show, do a ride and then a lunch. You get the idea. Find ways of getting into air conditioning. Make sure they stay hydrated. Use fastpass as much as possible. Don't miss the street entertainment. Be on the alert for restrooms; some how as you get older you need to use them more. Consider a break mid day at the resort. You can swim and your parents can take a nap. Stay away from the heat of the day in the parks.
Try to find out what interests them the most. Get the free DVD from Disney and have them view it. Use u-tube to show some of the rides and shows. Take into consideration any health considerations. For example, some rides are not meant to be done if you have a heart condition. Other rides are not good for those with back issues. My husband who loved roller coasters couldn't handle them after 70. He feels nauseated when he tries, now; so, it's very seldom that he does it anymore.
For some special activities that's a little dependent on the budget. Next door at the Grand Floridian Resort, the afternoon tea is wonderful. There are firework cruises for MK and Epcot. Doing the Disney Imagineer lunch is wonderful. You can have them do a spa treatment. Having some special meals like in Cinderella Castle is a good one.
Go on an adventure of finding hidden mickeys. You can find loads of clues here and on other websites for this one.
I think I would invest in photo pass and make sure you utilize it.
My 89 year old mother enjoyed the rides Soarin' and Buzz Lightyear and Toy Story Mania and It's A Small, Small World and the Great Movie Ride the most. She enjoyed all the parades and fireworks. The street entertainment was good for her, too.