I have considered driving out to a (reasonably remote) campsite, taking all my survival gear out of the car, locking the car (with my tent, sleeping bag, battery-powered combo TV/VCR, etc. inside) and then sealing the car key in an envelope. If you can complete the weekend without having to rip open the envelope, then you're probably doing okay.

I suppose the only caution would be if it's extremely cold - you might get so cold that you can no longer unlock the car door. (When my brother spent his first winter in Grande Prairie, Alberta, he discovered the hard way how quickly this can happen - he went out to start the car one morning wearing only a shirt (and dress pants and shoes). By the time he got to the car, his hands were so cold that he couldn't turn the key in the lock; by the time he got back to the house, he could no longer open the door, and had to kick at it until his wife opened it. I daresay in a real desperate situation, he would have been able to open one or the other, but he learned there is a world of difference between 40 below and what we grew up with in Nova Scotia.