Like the "learn scrum by cleaning your apartment" example? Give me a real world example of this working please. If it's worthwhile there must be a case study to build from.
Not sure _useful_ is a good metric, most useful things can be had more easily by buying it. And if its something you really need, then its better to avoid the stress of having to build it right and on time.
But go for something _interesting_, something _you want_.
A music instrument maybe? Automate the blinds in your apartment? Track the motion of earth for long-exposure photography? A multicopter for flying FPV?
