1. Make homemade ice cream with two coffee cans. You don't need a bit expensive contraption to make your own ice cream at home. Just mix whole milk, whipping cream and sugar in a one pound coffee can, seal it and put it in side a three pound coffee can. Pack ice and rock salt around the small can and roll it around on the floor until it freezes. Mix in fruit or other topping to make different flavors. There are hundreds of recipes available online.
2. Make your own Slip 'N Slide using plastic tarps. A store bought Slip 'N Slide is just a cheap skinny tarp anyway. Get your own tarps at Home Depot, and poke holes in a cheap garden hose. Then run the hose along the edge of the tarps to keep them wet.
3. Make your own flexible cold packs for injuries. A lot of families keep ice packs in the freezer, so they're ready if someone gets hurt. A store bought ice pack is 15 bucks. You can put water and rubbing alcohol in a Ziploc bag for about a dollar. The ratio is 2 cups of water to 1 cup of alcohol.