Ice packs are a great way to keep your lunch cool, but they're a bit expensive if you lose them. Cooking blog My Kitchen Escapades has a cheap and easy way to make reusable ice packs out of kitchen sponges.
All you need to do is take a standard kitchen sponge, soak it in water, put it inside a ziplock bag and freeze it. When you're done, you have an ice pack. The handy thing is that when the ice starts to melt, the sponge soaks up the water so it doesn't leak everywhere. If you're looking to upgrade work lunch, this should be a helpful trick.