Spot-Futures arbitrage, especially if your brokers allows you to collateralize your futures position with the profits from your spot position which would allow for higher leverage on the futures side.
Let's say the futures price is higher than the spot price and there is 3 months left until maturity. You sell the same (USD equivalent) amount in the future (expensive) and buy in the spot (cheap). You just made a profit and no matter where the price goes - you're hedged. The only thing is that you're stuck with 2 positions now. Just wait 3 months until maturity and the futures and spot price will converge to the same price. Now buy in the futures and sell in the spot and you've done it!
Of course, while being pretty much risk free the upside is also limited to how much (percentually) the future is above/below the spot.