My idea was to go all the way back to the creation of the universe. (bring oxygen) Then watch all the stars etc being formed. I would have a video camera to have proof and also to rub it in peoples faces. And to make scientists start all over on their theories. Then on day seven i would hang out with god while he rested. After Adam and Eve are created I would make sure they don't mess everything up for the rest of us. When i hear that snake in the tree start to talk, i would swing him against the tree, and yell, "SNAKES... CAN'T... TALK!" repeatedly, and hit the snake on the tree during every syllable... so he understands. Then i would go back to enjoy my immortality...
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...but its not like i sat around, like a loser, thinking of all the details of what i would do or anything...nope