The safest method IMO is poloniex btc loans, you can easily grow your btc by around 30-50% with little to no risk. Investing in all other things comes at a great risk and you have to make sure you know what you are doing first.
Growing your bitcoin instantly or in a very short time is tedious. As mentioned to be on the safer side it's good to invest with the reputed exchanges though they provide small amount of profit for the investment. You could get a sustained earning through this rather than getting trapped into scams.