We usually think of 7-Eleven when we think of slurpees, but there is an easy way to make one out of any cold drink. Teaching blog Charlie Brown's Teacher has a recipe that only requires a freezer bag, salt and water.
Mix the salt and water together in a heavy-duty freezer bag, shake it until the salt is dissolved, and put the bag in the freezer overnight. The salt will lower the freezing temperature of the water which means it will take longer to freeze, but that also means it will be colder when used in your drinks.
Once you take it out of the freezer, put the frozen saltwater bag in a large freezer bag with two cups of any sugary drink. Shake it for a few minutes to allow the frozen saltwater to freeze the drink into a Slurpee-style concoction. The recipe notes that batches that are larger than two cups will need a better container to stop the saltwater from leaking.
Science and Slushies [Charlie Brown's Teacher]