Warning: For all of the methods in this post, make sure you gather the rocks from a high and dry location, as waterlogged rocks can explode dangerously when they heat up in a fire. The steam builds pressure in the rock causing it to blow up like a grenade. Also, avoid slate and shale, as they are prone to explosion regardless of where you find them.
A large flat rock can be set up over a fire like a griddle, as I wrote about in an earlier post (link to hot rock frying Oct 2011), but a faster rock-fry method is to throw a flat rock directly in your campfire to heat it up. Using a stick, slide it out of the fire after 30 minutes. Dust off the ashes, grease the surface a little bit, and fry away. Bacon is great this way, assuming you brought bacon from home. It lays flat and doesn't curl up too much. Reheat the stone after frying a few things, and you can use that rock many times over.