A pedal-powered ice cream machine that requires you to burn calories to make the ice cream. If you are going to eat it, you might as well earn it first.
The machine would be configurable based on the quantity you put in. You have to burn at least as many calories as the full portion of finished ice cream contains. If multiple people are going to share the ice cream, they can share the pedaling too.
Other than pedaling this could be made into a device that is powered by any muscle groups that can be trained for endurance. For example rowing, stepping, cycling, hand cranking, etc.
Other than being powered by human-made kinetic energy the machine would function just like any other home ice cream maker.