I think you'd need to be more specific on what you mean by "cool stuff." If you want to know how to do something inparticular, we can tell you, but we don't know what you want to be able to do.
You could have the character pick up certain items in the game, then go back in time and be able to use those items earlier on, or you could have certain bits of knowledge that the player picks up and use variables to record when you get this information, then go back in time and have a new choice that only appears if you know that bit of information.
