M.G.
You are so lucky! My kids want to go to NYC! that to them is the ultimate trip at the moment. They want to see hockey, and see the sights (shows) that sort of thing.
So here are some thoughts based on my kids and nieces and nephews over the years -
water park would definitely be up there
you've done the skiing if I recall
what about the outdoor theme parks - maybe you did this with the skiing - like the tubing down the mountain and they have water parks with them inside, but skating outside too ..
Maybe that's too big.
My brother got memberships to things - like science museum, or something like that he would enjoy?
Is he into trampoline parks (mine were at that age)
Is he into skateboarding (mine were..)
What about those virtual things they put on their heads (can't think of what they are called)
If he's into seeing a hockey game .. (or other game) mine would have liked that at that age
Hard to say without knowing his interests.
I say though the water park - hands down would have been loved by ALL my kids at that age.