Sounds great. I developed a similar idea (in special school - very mixed ages) with the aim of teaching particular communication skills. Students chose their half termly activity from a selection - anything from junk modelling to gardening to parachute games - and all had a couple of communication targets to work on during that activity. Perhaps in mainstream you could have social skills or learning skills targets to cover. Or preferably, you wouldn't need anything like that, as the purpose of the activity is obvious! But for convincing heads/ timetabling and subject coverage maybe you'd need it.