Make a reservation for the entire party at a nice family sit down place for breakfast or dinner or whatever. (Like Whispering Canyons maybe? Or even Crystal Palace or whatever your taste is). Then during the meal, the server can bring them over and SURPRISE, Gran and Gramps have somehow joined you for a meal!
Likewise, they can all of a sudden meet you at a favorite ride, or show up at the door of your hotel as "housekeeping", or meet you SURPRISE at the monorail or bus.
You can plus it with having them bring favorite treats, balloons (don't release balloons, it's illegal to do that in FL), pins, all sorts of things.
I always think of those sorts of surprises as the best. Like the Folgers coffee commercials where the kid shows up from college as a surprise and the mom gets all weepy. That's just awesome.