DIY reusable cold packs that stay cold for several hours.

We made a bunch of these before I had my last knee surgery (in June! When we still lived in a house with only a swamp cooler!). That's now more than 10 years ago, and the ice packs are still going strong. Spouse used them last year on a broken bone. They've lasted longer than the ones my previous physical therapist sent home with me, and those were damn good packs.

Ingredients:
A pack of water beads or water crystals. (Find them at the craft store with the floral supplies)
2 bottles of 91% isopropyl alcohol (70% (rubbing) alcohol also works, but they defrost faster)
Water
Some quart zip bags
(Optional) vacuum seal bags and a vacuum sealer

Put a tablespoon of the dry water beads in a large bowl (3+ quart) and add 2 quarts of water. Wander away and let the beads get big. One packet of beads should make several quarts of hydrated beads, so if you've got a large dishpan or tub, you can make more at a time.
When the beads have some size, scoop about 2 cups of beads into a zip bag, then add about 1/2-3/4 cup of alcohol to the bag. You do not want the bag overfull, but at a nice size to lay flat. The alcohol lowers the freezing temp so the bags stay flexible, but also stays cold.
Zip the bag mostly shut, squeezing out as much of the air as possible.

If you do have a vacuum sealer, put the zip bag inside a vacuum bag and vacuum seal, then freeze. Round the sharp corners so nothing's pokey. If you don't have a vacuum sealer, put one bag inside at least one other bag, with the zippers on opposite sides, then freeze. If you've got little, inquisitive people who might like to take apart the bag, use a piece of duct tape to seal over the outer-most zip. You can use the colored water beads if you want -- the purple ones are pretty, the blue are great for color coding.

You can make the bags any size - I use quarts because that's just what I buy, but you can make gallon sized or snack sized ones that will fit nicely in a bra or sling around a neck inside a bandana. Cost should be around $7 for many, many bags if you don't have any supplies -- $2 for the beads, 2 for the alcohol, $3 for bags. They work great in coolers, help regulate freezer temperature, and if you need a warm pack, they can also be warmed in some hot water.