I ended up sticking with the monetary payments on my DO referral account but I also earn credits (they don't offer cash) from Linode.

With Linode, I keep a small box there with my smaller sites and just let the credits pile up around it. I consider it a rainy day hosting fund, if anything I'm running blows up, I can scale up without worrying too much about the immediate economic impact.

If you're feeling a bit more altruistic, perhaps you could find some open source projects that need documentation mirrored or a not-for-profit that you could donate a droplet to?