The ones in your mind I think is the ways in which you will not spend any capital on it but you'll have Bitcoin or other cryptocurrencies otherwise, if I'm not mistaken. Earning Bitcoin via Aidrop is also a good way on how to earn it besides on signature campaign, however, I can assure you that most of the airdrops right now are just garbages and won't even give you a decent money.

The best way to earn Bitcoin is by working, either on real life (like getting a job) and use your salary to buy Bitcoins, or work here in this forum, for example is a campaign manager or join a bounty.