First, you need an awesome George Foreman G5 Grilleration, a small appliance that the female members of my family think sounds like it should be a sex toy.
You don't really need a G5, but a grill that closes might cook the potatoes better before they burn.
Slice the sweet potatoes into about 1/3-1/2" thick slices, but try to make them all the same size for maximum grill marks.
Mix some olive oil or other oil with salt, pepper, freshly grated nutmeg and cinnamon (How much? I wasn't paying attention. Don't go overboard on the nutmeg. Put plenty of salt.)
Brush both sides of the slices of sweet potato with spiced oil and put them in your grill. Cook for how long? I wasn't paying attention. Maybe 5 minutes and then check. I took them out before they were quite done and put them on a plate, sprinkled them with salt, drizzled them with honey, and covered them with foil while the other batch cooked. That seemed to finish the cooking.
I didn't even peel the potatoes!
Cheap, gourmet, full of fiber and vitamins. What could be a better fall treat? Candy corn!