Mainly trading or gambling. But if you want a way to earn bitcoins without those risks, you need to do like you earn physical cash: Working.
Sell products for bitcoin, or create your own business.
well besides gambling and trading there are also so many ways to earn bitcoin, also gambling and trading involve so much risk and the lose of money is compulsory in trading and gambling, however the most easy way to earn bitcoin is signature campaign which is very convenient and without risk.
Agreed that earning from signature campaign is easy and safe way to earn bitcoins but you will never make huge income out of it as the payments are fixed in it and you will never earn beyond that limit and trading is surely risky way to earn bitcoins but with higher risks you will also gain higher rewards if you invest at the right place after doing good research.