Since Munchkin has been in the NICU I've been pumping breast milk and bringing it to the hospital in a cooler. However, more times than not I forget to put the ice packs in the freezer so I have to improvise.
Frozen peas and corn work the best because it's small and has little pellets that form to the milk bottles. Broccoli shouldn't be the first option because it makes the whole cooler smell like...well, broccoli. And definitely don't use ice in ziploc bags because the ice melts and the bags leak all over the place.
Word to the wise, don't forget to put the ice pack in the freezer!