royalestel shared the instructions for making your very own DIY ice pack. Check it out at How to make a $20 ice pack for $2.
If you are not inclined to click through to his awesome 'ible, here's the basic premise:
Dribble a cup of alcohol into the diaper.
Then soak it in water.
Slap it onto an aching body part or into your lunch box.
The alcohol keeps the water from completely freezing. The sodium polyacrylate keeps the liquids suspended in a gel. And you're only out a diaper and a a freezer bag. But you gain temporary relief from pain or warm beverages.