I've taken my parents down twice in the past few years and I'm taking them again in January (for 12 days this time). They'll be 75 and 78. We rent scooters for them so that distance and stamina aren't issues. They LOVE it down there, love doing the parks, love relaxing around the resort (we stay at BCV when they're with me), and they go on almost any ride except the thrill rides. Big favorite - Soarin'
I will say, though, that the rides that have continuous loading systems (i.e. "conveyor belt" loading) make them nervous. They prefer the rides where the cars come to a full stop for boarding and unloading.
Shows in general are a big favorite for my parents. They always enjoy Beauty and the Beast, Lion King, Little Mermaid, Indiana Jones Stunt Show, Fantasmic, Illuminations, Wishes, and Spectromagic. In January, I'll take them to the Nemo show and Lights, Motors, Action -- neither of which they've seen before.
They also love anything that brings back old memories, especially in MGM. So we go on Great Movie Ride, Backlot Tour, visit the Walt Disney exhibit (the name escapes me at the moment), and we eat at Sci Fi and 50s Prime Time sometime during our stay. They have a ball, and I treasure every moment with them.
You can always enjoy the "out of the way" places like the British gardens in Epcot, or the various scenic trails at Animal Kingdom. There are also great street shows around Epcot, like the Chinese acrobats, the "candy lady" at Japan, etc. etc. It's also fun just to take a boat ride between MGM and Epcot, with no destination but just to relax and look around.
We always take a break and go back to the resort for the afternoon. Then I get pool time, and they read the paper, have coffee, take a nap, poke around the gift shop, whatever they like. Then we go back to a park for the evening, feeling refreshed.
Have a wonderful time!