Una,
for me, the key way to bring science thinking into this creative activity is to do exactly what you have suggested:
What would the Alien need to survive?
The child who creates a wonderful, imaginative alien can then extend the activity by describing the ecosystem that they live in.
When we make aliens (using tin foil) there is often a child who gets a bit discouraged and crumples theirs us into a ball. I suggest that this is an alien in camouflage, waiting to catch an other alien by surprise!