I'd like him to do something new that will engage his brain or body, but that he'll enjoy. And not just killing it on PS games 
Ideas I've had so far are -
Learning a new language - maybe Makaton or one with a different alphabet, like, say, Greek.
I'd love him to learn to bake cakes, treats for us all and I think he'd be on for that - but where to get the flour?!? Sadly, he wouldn't be as keen to learn new meals and take over feeding us (although I do insist he learn to make one new meal every school holiday).
Practising to try to do the splits or a difficult yoga position - while he's still young and flexible 
Any ideas? Is there anything you can recommend that your children have tried, or that you are doing? I'd like to have a few suggestions to make, so he can choose.
I've been struggling to think of things, because he'll have to do them at home and it depends on whether we can get any equipment needed to do it.