My mom and dad are seniors (78 and 82) and at MK they like It's a Small World, Jungle Cruise, People Mover, Carousel of Progress and the parade and shows. Plus, they like to walk around just taking in the sights.
At DHS they like The Great Movie Ride, the Backstage Tour, Indiana Jones and the Streetmosphere people (or whatever they're called) and One Man's Dream. They get a kick out of 50's Prime Time and Sci-Fi.
At Epcot, they always go on The Land ride and my dad loves Test Track.

The last time my mom went on Spaceship Earth, the part where you go backwards hurt her back a bit so I don't think she'll do that next trip.
They like just walking around AK and they find it relaxing.
For dinner, they really like the signature restaurants as they tend to be less noisy.
I think it's fantastic when senior citizens go to Disney World and can be kids again. I hope I'm healthy enough at that age to still be going!
I've warned my mom and dad to be careful walking around. On our most recent trip I saw at least three elderly people fall, mostly when they didn't see a curb. I saw someone else fall off their seat on the Disney bus.
Just take it slow and have fun!