Hmm. I think it is more complicated, because for kids the boundary between play and other activities is less well defined. If an adult built a sandcastle, it could be play, but it could also be done for art or to enjoy the act of creation. In that case I'd say it was not play. A kid could do that in theory; but in practice they usually aren't doing it solely as art.
I don't think age has anything to do with play, and in particular if we just confine ourselves to Caillois's theory about play:
It is free, or not obligatory.
It is separate (from the routine of life), occupying its own time and space.
It is uncertain, so that the results of play cannot be pre-determined and so that the player's initiative is involved.
It is unproductive in that it creates no wealth and ends as it begins.
And it is one of either make believe or governed by rules separated from normal life, then it is play regardless of whether it is doen for art or to enjoy the act of creation. Unless the adult was making the sandcastle professionally, then it's definitely play under the above observations. Of course, this raises the question of whether you can be paid to play (a professional player) and I would argue "yes", for example a soccer player, and thus Callois 4th rule should probably be translated better that it creates no capital goods. For while you can get rich as a soccer player, you get rich by entertaining others not by making anything of inherent or practical value.
Thus, again, I conclude that you can play to enjoy the act of creation and Callois is largely right in defining what play is, but largely wrong when cataloging the reasons why you might play.
Is the discovery part of the game or incidental to it? If you take a helicopter to the top of a mountain, you get the same discovery and beauty. But it isn't a game.
Now, that's an interesting question. If you remove the challenge from the play of mountain climbing, and leave only the discovery and sensation, is it still a game? It's free in the sense of you are under no obligation. It's separate from the routine of life (if you aren't the pilot giving the tours). It's unproductive in the sense of creating no capital and leaving you with nothing but memories. It's uncertain in the same way the mountain climbing is, in that bad weather might cancel the flight or landing at the summit. It's not clear however that it involves any make believe or any rules differing from the normal routine of life, but neither for example does tree climbing which I think we will want to insist that in the case of a child is a form of play. So I'm not sure. I think since I'm inclined to say tree climbing is a form of play, that I'm more inclined to say Caillois definitions are just too restrictive regarding what play is and it's more likely than not that the helicopter ride is a form of play as well because I really don't see a reason why it shouldn't be said to be play.