In my opinion the best investment opportunity is to just buy and hold bitcoin. In the long run I think you'll likely see great gains. Other investments are too risky and offer little return.
It really depends on your risk appetite.
For those enjoying risk, you can lend your bitcoin or invest in gambling sites. But you should diversify your investment, and never lend/invest more than you can lose.
For those hating risk, you just need to put your bitcoin in a paper wallet, and wait for the bitcoin price to rise.