1) Honestly, the affiliate idea shouldn't even be on the poll, as that should always be available as an option, if you can find those willing to support (like Path Network).
2) I agree that if you were to write a good guide for the cryptocurrency, including which sites you consider reputable enough to work with, would be a good option.
3) Direct covering of servers would be good as well, since it covers the direct costs. Only issue is knowing that someone can keep it up.
4) Ads, while many support the idea of non-obtrusive ones, should really be a last resort. Even ad-blockers are not necessarily able to block all of them, and if they aren't triggered, you may not get income from them.
You can add me to the list to assist with server costs if that is the route you go, or contact me through the notification system if a more "direct" donation is needed per monthly basis.