According to me The best ways to grow your BTC is by trading. You can multiply your principal amount in few days of trading and your profit margins can be terrific. But remember that trading is a risky and unpredictable job.
You can also grow your Bitcoin by investing it in various new projects.
trading is one of the best eay to grow your bitcoin. But trading its not easy you need a lot of knowledge first before you get that or to earn bitcoin. I dont think few days and your money it will become double maybe 2 weeks or 1month.
to me I will also select trading, there is no doubt that trading is the best way of making double our investment. but may be there are some experience gambler who can also make good profit from bitcoin and they can even doubt their investment in a very very short period of time. currently those people who will even invest their money in bitcoin and will hold bitcoin for some time they can also make their investment doubt because there ate more possibilities that the price of bitcoin cross 5000 $ in next few months.