Found this recipe in Parent Hacks, a really cool weblog for parents. The about reads “…everything they left out of the instruction manual. oh yeah. there is no instruction manual” :). I already love the site 🙂 🙂
Fill a pint Ziploc bag with 1/2 cup milk (I suggest whole milk), 1 tablespoon sugar and 1/4 teaspoon vanilla. Place sealed pint bag into gallon bag and add ice to fill the gallon bag halfway and 6 Tablespoons salt. Zip (and maybe double bag it), then shake, toss and play catch for 5 to 10 minutes. Kids might want mittens!