You can earn bitcoin using anything, it depend on your creativity and skill.
For example, you have high speed connection or huge internet bandwidth, you can offer leech, proxy or download upload using your internet and they pay you using bitcoin.
Yes, we can earn in different ways that we have skills, like if we do altcoin trading , then it is best source to earn bitcoin. Although we also earn bitcoin in a free ways, like faucets, and also do some work in signature campaign and earn bitcoin. But best way i will be say altcoin trading for bitcoin.