I haven't explore that much in bitcoin, but I have tried mining and currently on a campaign. I have little ideas about trading and gambling, but for me, I would prefer trading and campaign. In mining, you need a high end computer in order to mine coins, you also need a fast internet connection and probably a source of electricity that will not cost you a lot of money. Gambling as we all know is hard to compete with. People involve in it are high rollers, and some are war freaks. Meaning, if you get to bump to them, expect that death is at the tail of your trail. That is why I prefer trading and campaign. Although its a long process before earning, still its safe. Just no to scammers.
Except you just want to be mining some new coins on the block, then a PC might do, but for what it is worth, you will actually need mining devices such as ASIC, GPU and co, to be able to get the best from mining and they sure come with a price, and also come with a huge price most especially if you want to have a big set up before we would even start talking about electricity. So far, Trading and campaign still remains the easiest for anyone to be a part of and for me, I started with campaign, during these period, learned how to trade, and now I combine it.