A safest way to invest the BTC is to keep hold of the bitcoins you have till you see the price increase. I think this would be a good idea rather than investing on trading, mining or on anything else. Another way would be, if you are skilled on designing or completing projects, you can hire people and invest in them and double your btc. This would bring you a decent income.
Only keep and hold bitcoin in long term is the safest way. But we have the point that we need to wait in really long time. it is fine if we saving big amount bitcoin becouse the profit will significant. I think the best way is we work smart, I mean if we have a good skill in any profession we can use it to earn bitcoin. For example, we sell a clothes then we get the payment in bitcoin. Although ee have to convert it into fiat first to buy some thing. I think trading, or minning is for individual investment although there are some trader or minner are work together.