This shows you how to make a dish soap ice pack. This is something I found elsewhere on the internet, but I though I should post it here because I've seen several attempts at making ice packs that didn't seem easy or convenient.
Step 1: Fill and Chill
Simply fill a sandwich bag 1/2 full of dish soap and chill in the freezer. (Double-bag it to keep it from leaking) Done.
This picture shows Dawn being used, I don't know if other soaps will work in the same way.
Some other things that I didn't state that were brought to my attention:
1. Original website WiseBread.com (For those who asked. The picture is from there too, I hope this doesn't offend anyone).
2. The reason you use dishsoap is that it will get cold and still flex around joints and such.