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.
Well for me it is still good to save bitcoins not only for holding it and wait for price increase you can also save for buying your own hardware miner.. so that you can mine bitcoin without too much work.. and you can use them also for trading so this combination trading and mining is good and you can be make a good profit.. Well we are here online so if you have some free time you can study some online jobs like making website javascript pythons or any programming language.. for me i study SEO and online marketing i study started 2015 and and its my part time job and passive income i am making sales in my own website. and earn money. some of my money converted into bitcoin because it is always good to buy bitcoin to make more profit..