Hi Janet,
There are lots of exciting things for your 5 year old son to do outside the parks, here are a few of my suggestions.
The Grand Floridian Resort offers several lovely packages for children, how about booking him on a Pirate Cruise or if he's a budding chef how about the Grand Adventures in Cooking both listed
HERE.
How about taking a trip to the Lego Store at Downtown Disney, I'm sure he could spend hours there playing with the Lego or he could make his own treat at Goofy's Candy Company. If the weather is nice you could show him the fountains to play in near to the Disney's Days of Christmas Store, bring a change of clothes though as he will get wet!
As you say Mini Golf is a great way to spend time outside the parks, they are fun for all ages and I'm sure your son would love them. I would suggest the courses near Blizzard Beach, Winter Summerland, as my family find them slightly easier. They have child size clubs and my children normally beat me on several holes so be warned ;)
Whatever you decide have a fantastic trip,
Jo