We know that four way to cryptocurrency income such as Invest ico, mining, trading and Airdrop-bounty. If you wish, you can do any of that.
If you want to mining or invest in ico and need a lot of money, it has cost-effective risks and have also profits. And if you want to do trading then you can start trading only if you have some money. Here's a little less risk and have also profit.
If you do not want to take a risk then you can start the bounty. There is no risk here only you will have to work on time and you will get your payment at the end of the job. As a payment, you can get cryptocurrency and convert them into meaning by selling them. So, I think Bounty and airdrop is very good for earning risk-free cryptocurrency profit.
There is actually a risk here and that is the risk of not getting paid which is something bounty hunters can relate with very well and because of this, the risk embedded in this course of action is not too different from other options you have highlighted above and some times could be greater because while in others, you know the risk and there is huge assurance that the amount you will gain is coming for certain but in the case of bounty, the chance of zero bounty is higher than others as the bounty might be a scam, the project failed to raise the required milestone thereby necessitating a cancellation or even being paid but the payment is worthless eventually couple with the amount of time you have to wait to get paid in the first place.