How can I cool a six pack while out on a picnic?

This question appears very ambiguous and I am assuming that the 6-pack is warm, there is no ice available, and you don't have a cooler.  If that scenario is accurate, the only idea I can come up with is to find a nearby stream that is flowing relatively fast and submerge your 6-pack for about 15 to 20 minutes.  If there is no stream available I am afraid you will most likely be drinking warm beer... unless another well equipped picknicker with a cooler is either willing to trade you cold for warm or chill your beer while you wait!

If you're near a lake, put it in the water.  If you're near a cave, put it in the shade.  If all else fails, bury it like a ground hog.

If you have petrol or better yet methylated spirits place your beer in a sock, soak the sock with the meths and then swing it around until dry the evaporation will have cooled the beer.  Works better with aluminium cans than glass,