1) Lend BTC on a site where you can directly lend your funds to the borrower like BTCjam
this is a lot of work a lot of risk gaining some profit. finding a trustworthy person to lend to is hard and the amount of btc that you loan is usually low if you want to minimize risk so the profit is low too.
2) Sell your service if you are good enough in something you can make a thread here on forum and make easy bitcoins.
this is the best way of earning honest money in form of bitcoin. and you can earn the most this way.
3) Buy altcoins with bitcoin and do trades.
you have to learn the tricks of trading and lose a lot of money to master the market. so be careful if you choose this option
4) Cloud mining -like cex.io
did they resume their service and is it still profitable to mine on cex.io?
5) Gamble your bitcoin to grow fast.
NEVER.
you should never consider gambling as a way of earning profit on your money. because you will lose your money guaranteed.