All You Need Is Just a Sponge and Some Water
Gel-filled ice packs are the best option for several reasons: they don’t melt and make a mess, plus you can reuse them. They’re also very affordable – if you have any spare sponges at home, you can make your own gel-filled ice pack!
Put a (clean!) sponge in a zip-lock bag, and fill the bag with water. Seal it, and place it in the freezer. When you need an ice pack, put the sealed bag inside a thin cloth and use it as needed – they can be refrozen multiple times!