There was at least one Game Boy cart with fun features: Aprilia had a GBC cart[0] that had an interface cable for diagnostics on one of their scooters: [0]: ht

There was at least one Game Boy cart with fun features: Aprilia had a GBC cart[0] that had an interface cable for diagnostics on one of their scooters: [0]: https://youtu.be/bknfVpctoSY?si=_neVHq_fkJRfDH5I https://youtu.be/bknfVpctoSY?si=_neVHq_fkJRfDH5I