I would go with invisibility. Time travel can mess stuff up.
EDIT: I'm glad this is back. Many memories in this thread.
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Time travel. It'd be cool to visit all these famous events and places that you learn about. Also you could screw with people. Pregnant woman: We don't know if it's a boy or girl. We want it to be a surprise. Me: It's a boy. He becomes a lawyer. Dies in a skiing accident. Favorite color is indigo.