I've created (and use daily) a project that uses Minetest at its core. It's a game system I've built to take the boredom out of indoor exercise. You can try it if you like, my additions are all released under the GPL. Some assembly required...
Minercise is a silly name for an exploration and survival game I've created using the open source Minetest sandbox building game and some exercise equipment I've adapted for use as game controllers. I have a recumbent bike mounted on a stationary trainer, an ergometer (rowing machine), and a treadmill all of which have been modified to use for the game. The modifications were achieved by designing and adding sensor systems to each of the machines using Arduino and compatible (Pololu A-Star) microcontrollers. It was also necessary and desirable to modify the game code to function with and enhance or restrict the functionality introduced by these controllers.
For better or for worse I did make patches* that modify the Minetest game in order to make the controllers work better.
Many many thanks to Perttu and all the Minetest developers for making such a great project and releasing it under the GPL!
Adding to the idea: Automatically donates $10 to Minetest each time you don't fulfill a specific distance goal, e.g. 5km, within a specific time, e.g. 30 minutes.