We take our bikes all the time and just put a lock on the frame and wheel at night and lay them on the ground near our trailer tent. We have never had a problem. You could also lock them up against a tree trunk.
Are you thinking of a dog spike that screws into the ground? We use of these to secure our fridge on dodgy sites.
For bikes we usually lock them together and then to other things so either the towing eye of the car, or a fence, tree or whatever else. If there isn't anything solid we lock them up against the gas cylinder with a couple of saucepans and lids included as well - we work on the basis that we might not be able to stop someone taking them but we'll certainly hear them move them!
Mushinski,
if you have one of those expensive, posh mountain bikes with disc brakes, how about this?
a motorbike disc lock with an audible alarm if moved. Zena bike alarm/lock