Your handpacked lunch. Sore joints. Picnics. Having a quick source of cold is infinitely helpful for keeping what you need chilled, and doing it well.
As it turns out, you can make your own - infinately sizeable and shapeable - but just limited how water freezes by adding something with a much lower freezing point, like alcohol....both the rubbing and the, er, vodka kind.
TipNut has assembled a few great DIY options, as well as some excellent packing and energy saving tricks.