I like your snow analogy, zeusk. You go back, change something, and the changes are there as you go forward again, but by the time you get to where you turned back you're exactly where you were when you went back in time.

All sorts of analogies could be used. Some might even be useful for a game (rather than the traditional "Don't mess with the past!" approach that's so typical, let them mess with the past however they want because you're not at risk of disappearing if you prevent yourself from travelling back (river analogy), or there are game-mechanics in place to force everything back to where you left it by the time you reach the present again (snow analogy).

I think zeusk's original post was calling for more imagination/creativity than "Time travel is impossible", although I could be wrong.

Jibb