My parents have gone every year with us. They just meet us there, though, and don't stay for all our days. They usually show up after lunch on Magic Kingdom day and then stay for one more day. Over the past 8 years, I've noticed they definitely have slowed down and needed to take a lot more breaks for themselves. Their knees just won't let them travel as fast as we tend to go!
When I plan our days with them, I minimize the trips back and forth across the park. If we have some rides on the opposite side of the park that I know they won't like, I plan something else for them. For example, if we need to run to Buzz and Space Mountain, they can sit and enjoy the Hall of Presidents. Then they slowly stroll over to meet us for Laugh Floor. They don't mind splitting up from us for a bit, though. It was really nice to split up when DD was younger. They could take her for face painting or the carousel while DH and I would hit the big rides.
My parents pretty much like any show, and those are good places for them to sit and cool off while we go on the rides! Their favorite thing to do is just watch DD enjoying the magic! They can't do anything spinny anymore, but they enjoy all the Peter Pan, Haunted Mansion, Small World sort of rides. Toy Story Mania is probably their favorite.
Have a fun trip!