I cannot possibly top Rainy's pie. I can, however, attempt to make HER drool. Since I'm conceding the sweet category to her, I shall attempt the savory category.
(I love grilling out. A lot. The only thing I want for my birthday is a T-shirt with a picture of a grill and the legend "If You Can't Stand The Heat, Go Get Me a Beer." Well, that and a kabob basket.)
This makes enough potato goodness for four people, and as you'll see it scales well.
2 red potatoes, skins on, cubed
1/4 vidalia onion, chopped
Garlic salt, lots
lemon juice, enough to dampen the top layer
Mix everything in a bowl. Pull out a square of heavy duty foil, fold it in half, and loosely fold the sides in to make an envelope. Spoon in a fourth of the potato mixture and throw in a dollop of butter. (See, Rainy, I cheated too.) Fold the top in.
Take your foil envelopes to the grill. If you have already cooked the meat, bury the packets in the coals. If you haven't, lean the packets along the sides of the kettle, facing the coals, and turn them around every so often.
If you're using a gas grill, I ask you, why do you think you're grilling out when all you've done is move your stove out to the deck? NO POTATO PACKET INSTRUCTIONS FOR YOU.
The key to "doneness" is the smell. If you can't smell them anymore, you're burning them.