You can start giving out loans and charge 10-20% interest with collateral to people, even on these forums. That's a nice way to make some profit!
That can be one way to double your bitcoins.
I prefer Gambling, Gambling and Gambling, as I want to double it in a quick way, I would prefer to take a risk and even if I loose doesn't matter as I know that the day luck favors me, I can make it double or triple in a single day.