My hack is a necessity for hot-weather traveling with my son: a Frogg Togg Chilly Pad. These "cooling towels" are about the size of a kitchen towel, and wick off water/sweat on one side, while the gel side sucks up liquid and holds it in and cools you down.
We discovered these when we were prepping for our trip to Disney World. My son is a total heat sponge and, after two days of 95 + degree weather, had a wicked fever. We soaked the towel in cold water and carried it with us in a Ziploc bag. When his fever got scary, we ran the reusable towel under really cold water, and put it on his head and back to cool him off. The towel took his temperature down by several degrees and worked very quickly.
This is also useful to have if you've got a long drive ahead of you. Soak the Chilly Pad, squeeze it a little so it won't drip, then drape it around your (child's) head or neck. Hot cars have contributed to my son's car sickness (the air conditioning always takes a little bit to catch up), and believe me, this cools you down pronto!
Great item for a first aid kit, emergency supply kit, or camping kit. I could also see this making an appearance during flu season, and to help a kid get to sleep on a hot summer night.