Someone else has already mentioned a bicycle, but my vote would be for a bicycle with an e-bike upgrade kit.
Do the hard work upfront to find something compatible, in particular around batteries (so they don't e.g. start a fire). From there, upgrading a regular bike to an e-bike and routing all the cables / logic circuits / etc. is a project unto itself. At the end this kid gets a fun and efficient means of transportation, as well as the satisfaction of knowing how their bike is put together both mechanically and electronically.
Do the hard work upfront to find something compatible, in particular around batteries (so they don't e.g. start a fire). From there, upgrading a regular bike to an e-bike and routing all the cables / logic circuits / etc. is a project unto itself. At the end this kid gets a fun and efficient means of transportation, as well as the satisfaction of knowing how their bike is put together both mechanically and electronically.