Best bet for outdoor beer cooling in the adsence of a handy cold stream is an old washing up bowl and a towel. Lay your bottles in the bowl, wet the towel and cover the bottles with it. Put the whole thing in the shade where the air can blow over it. Evaporative cooling will keep the towel a few degrees cooler than the surroundings, certainly cool enough for ale-type beers. Lager fans really need ice, but that's their problem.
While camping in Arolla, Switzerland many years ago I went to do my laundry in the water-trough and found so much beer in there I struggled to fit the laundry in. There seemed to be a rolling stock of beer there whereby you dropped warm ones in and immediately picked up cold ones, and happily everyone was sufficiently honest to replace what they drank. :-)