Well... Now that i think of it there is a simulator game called Evolution which is pretty simple. All it is is you make a model of living thing with joints, bones and muscles and the game will give it an AI and you can try to get it to either climb well run well, or something else i think. But basically you make it and i think it evolves the design to better suit its task (running climbing, etc.). Not a sandbox but it does support a bit of creativity.
