Bicycles.
Ride rail trails.
Ride flat river bottom single track.
Ride gravel roads.
All are adventurous and a healthy 10yo can handle at least one of them(and likely more).
It's an activity that requires constant focus, improves balance and core strength, is aerobic, and potentially uses a ton of energy.
At easiest- ride trails or paths to a park or a couple parks he has never been to. Or ride to get ice cream or some lunch.
Harder- find MTB trails and work on balance, core strength, tempo, and gear decisions.
Even more challenging- map out some gravel routes and let him practice and improve in all the ways already mentioned, while building up endurance from distance. Ride to state parks or a lake or whatever for a break.
Make it fun. Challenge but accomplishable. Set goals and rewards.