1. Broken link building. It works really well right now, but as more SEOs catch on to it, the returns will diminish.
Here's a good intro on how to get started with it by Neil Patel over at Quick Sprout.
2. Creating content geared towards colleges. And then reaching out their relevant departments to make them aware of it. Nothing gives you a bigger boost in SEO than a few hundred authority links from colleges. Using this tactic I was able to create a books community that went from zero to over 500K unique visitors a month in less than a year. It requires more work and attention to detail, but the end result is worth the effort.