The most important advice I can give them is --- GO!
I agree with everything daringstoic said.
As for itinerary, remember the old saying that no plan of battle survives first contact with the enemy. Plan all you want but one long line can wreck your detailed schedule. As I am sure you already know, flexibility is one attribute that help make a WDW trip successful.
As for descriptions of things they can do, I suggest that you get a copy of "the Unofficial Guide to Wald Disney World 2018" by Sehlinger and Testa or "Birnbaum's 2018 Walt Disney World: The Official Guide" and give it to your dad. Both of these books are excellent and provide a wealth of information. Reading the World Showcase section may be all it takes to convince your mother.
Another great planning tool is touringplans.com . It costs around $10 or so to subscribe for a year but I think it is well worth it. You can plug in what you want to do. You can plug in parameters like "minimize walking" and let it generate a plan for you. We have never had this work perfectly (see first contact comment above....) but it will give you a plan to aim at.
One other thing to consider is their overall health. My mother, who will be a month shy of 89 when we take the whole family again this June, does well over short distances but cannot handle our average of 6 miles a day of walking at WDW. We take wheelchair and that way can cover about 3 times as much territory and she can enjoy it without being totally exhausted. (FYI she rode Seven Dwarfs Mine Train. Said it was two trips in one. Her first AND last.)
Finally, I love your dad's attitude. I still feel like a kid every time we go, and I am almost 67. (70's ain't seniors any more!!!
) My wife and I plan to go back to WDW for our 45th anniversary next January for a whole week. JUST US! It is a completely different trip with just the two of us.
Tell your mom to start packing. Tell her she can take her tablet if she wants to, but I'll bet once she gets there she will be right in the middle of things with the rest of you.