The venerable tin foil dinner is something that probably anyone who has ever been in The Boy Scouts of America or anything comparable (Cub Scouts, Girl Scouts, 4H, etc) has probably been exposed to -- or alternately, punished with. Tin foil and aluminum foil are used interchangably from here on in.
It is, in essence, the outdoor equivalent of a TV Diner. It requires very little prep time and is very hard to ruin except by inattention to cooking time or too much cumin.
An acceptable Tin Foil Dinner is usually just a basic meat and potatoes bit, with my personal favorite being a smattering of cubed red potatoes (with the skin left on, of course), sliced onion, carrot, celery, and even some kind of meat if you're feeling carnivorous; if you're going veg, things like corn, peas and beans (especially black and kidney beans) are excellent, though expect them to be little better than mush one you eat them, like what happens in slow-cooked stew. M_Turner recommends Water Chestnuts, too. Toss in a smattering of spices, especially course black pepper and salt and maybe even a little butter or olive oil (though oil goes rancid very quick). There is so set recipe other then choose stuff that you think will taste good together.
But the real soul of a tin foil dinner is, of course, the wrapping. Yes, there is a method to this madness. The foodstuffs themselves are put into a packet of aluminum foil (try to find real tin foil these days), enough to wrap and fold tightly around them, making sure that no juices from the food will leak out. Next comes the secret of the whole kaboodle -- wrap this first package in at least two layers of moist/wet newspaper, again wrapped tightly. Then finish the while thing off with another layer of tin foil, maybe double wrapped, around that, again making it close tightly. Store it whole thing in a refrigerator until you leave for your night of camping.
The moist newspaper does double duty here -- first, it helps insulate the food and keep it cool long after it's been moved from the refrigerator. Second, it helps keep the food from burning and provides and even raising of the internal temperature once the food is heated.
And how how does one heat a tin foil dinner? Easy Buckaroo, you just toss that metal boy into the hot coals of a campfire for 20-25 minutes or so. After 20 minutes of that much direct heat, the meat will be cooked and the heavier vegetables will be nice and tender.
As I said, there are few ways you can ruin one of these-- the easiest is keeping it out of refrigeration too long; if it had meat or oil in it, will spoil in under 8 hours, though strictly vegetable fare will probably survive much longer. And of course the other way is cooking it too long. If the foil is actually begining to have holes burned in it it, you've ruined it. But, just because it's burned to a cinder, doesn't mean you can't eat it; after all, you've eaten flaming marshmallows before.
Let it cool down and then go at it. Perfect food after a day of earning your Orienteering merit badge. Remember, pack it in, pack it out; leave nature better than you found it.