I'm surprised no one has mentioned gambling yet. "Just-Dice" is a fair spot to play.
Other than that - Investing, Lending, 'Shorting' (converting to Fiat), and buying up Litecoin are all good examples.
Gambling will generally lose you coins, not earn you.
I mean, I suppose you could be like "ok, I'm going to martingale ONCE" and be practically assured a profit, but if you take into account the minute chance you will lose everything, your expected return is still negative.