Trading, join a campaign or bounty and mining are the best one, gambling is not good to earn money because it's very risky to do and also you have a high chance to lose your money/coins too. If you have so much money, i think trading and mining are good to spend your money into it. You can get a nice profit by doing that two.
Based on my experience, trading and gambling is close enough because you can lose money too. I still prefer earn money by campaign, like sign campaign, bounty etc.
I don't say that trading is gambling but better learn a lot of trading technique or you just spend your money to "gambling-on-trading"
I choose trading and signature/bounty because these two things will give a chance to earn money. If you can learn deeper about trading, then you can get big profit in monthly and maybe you will use trading as your main source to make money. I think as long as you can buy at low price and sell when it's high, you will make money, and you don't have to think that trading will become almost the same as gambling.