You are asking about minimum risk investment method and yet you posted this in the Gambling section? You must know that more risk=more potential profit.
So ask yourself how much interest you want: 1%, 5%, 50%? And what type of services you like to invest in.
Do you want to invest in gambling services (for example: MoneyPot), lending services  (i.e. BTCJam),or maybe some unstable HYIP (not recommended)?