The Saturn Game by Poul Anderson - freeform roleplaying by deep-space explorers gets them into trouble. This was a weird story, but it won a Hugo if I remember correctly.
Animorphs: The Ellimist chronicles 
Ah yes, the aspect that the ellimist’s are a race of birds is quite interesting and that game that they played where they would create a replica of two planets and with as little change as possilbe see which planet would dominate the other, that would be an awesome game to play.
That’s… an interesting way to go. Never read Animorphs, so thanks for bringing it up. (I’ve read “The Saturn Game”, but it had slipped my attention. Thanks, xyphoid.)