Safest way to grow bitcoins is either hold them or trade with altcoins
Though trading alt coins is not a really safe way of growing bitcoins but it is obviously one of the best ways of growing bitcoins because it can earn you some good profits everyday if you a good enough capital and enough knowledge about trading.
That's true not the safes way but a good way to multiply your coins and if you can manage to invest good capital in it then you can make profits even from small fluctuations on daily basis.