|Peanut butter and Jelly frozen pops|
The simplest cold treat to make I think are popsicles. All you need is some fruit juice (or other beverage) and some molds and sticks. I have been able to find molds for popsicles all over the place, from the local dollar store to my local major chain store.
To make you simply pour some juice into the molds, add a stick and freeze until solid. To step it up a notch, you can add some fresh or frozen chopped berries, or blend them all together to make fruit pops.
Making what we call smoothie pops is also a popular activity in the home here. What I do is take our favorite smoothie recipe and (strawberry banana, here) and freeze it into molds like the juice.
To make the pops melt more slowly you can add unflavored gelatin (or flavored as well if you prefer) before freezing.
You can also take different flavors of packaged instant puddings and make frozen pudding pops, or yogurt and freeze into yogurt pops.