A great activity in the Summer, if you’re not afraid of heights (like me) is climbing through the trees up at Cote 2000 in Megeve.
There are tracks for all ages and our kids have been doing them since they were 8.
http://www.cote2000aventure.com/
And if you like this sort of thing, it’s also available at the Pontet leisure park in Les Contamines, where there are plenty of other activities – including a beach by the lake, for anyone who is not.
If parkour is you thing you can now do it just 10 mins walk away by the new Praz sur Arly lake