Here's our easy sock trick to keep your water cold while you wander...
We tried freezing the bottles we usually keep in the outside mesh pockets of our backpacks, but found that condensation on the bottles wet the backpacks. And the ice melted pretty quickly in the summer sun.
But if you take a pair of socks and drop a bottle in each, and then double the sock back to make a two-layer bottle cover, you not only avoid the condensation, you insulate the bottle so it stays cold much longer!
We've only tested it with one pair of socks, mens' size 10-13, but we suspect that this kind (soft knit, mostly cotton) works best because it insulates better than a thin sock would.