The best/safest way I found so far is being a liquidity provider on bitfinex, but you can't
eliminate 3rd party risk. If bitfinex goes under all your bitcoins are lost.
The only risk free method I know as of now is running a darkcoin masternode and I think bitshares/bitusd has
something like 3% yearly returns, but this are not strictly bitcoin related and you still have exchange risks.
One option which will work like darkcoin's masternode is
joinmarket but its still in the works and it might take a while till it goes live.
Generally speaking return on investment is dependant on the risk you're willing to take. So if you're going for low risk methods then the returns
will be as low. But if your bitcoins are just sitting there and you can find a zero risk method then any increase is good.
The best method of getting more bitcoins is just buying more. Find ways to make fiat money, which should be easier, and buy bitcoin.