plastic popsicle molds
vanilla ice cream (with sugar or sugar-free ~ lactose free is fine too)
1 can Root Beer (with sugar or sugar-free)
Fill your popsicle molds LOOSELY with vanilla ice cream (about 1/2 full). Very gradually, pour the root beer over the ice cream. It will foam up over the top (and seep down through the ice cream). Give it about 10 seconds, then you can push the foam down with the back of a spoon (or scrape it off). Add a little more root beer, and repeat. I did this about 3 times before the popsicle molds were filled to about 1/4" from the top (and only used about 1/2 of the can of root beer for 6 popsicle molds). Freeze until firm.
I used 10 calorie root beer (for the whole can) and lactose-free, no sugar-added ice cream, so each little creamsicle was only about 35 calories (or less).