Creating a circle of links
You can create relevant links among your own sites even if they aren't on related topics by creating intermediate sites that are related to each of the others.
For example, I have a blog on veterans benefits. I have another blog on domestic travel. So to create some links I've built a small web page listing veterans museums around the country and linking it to both the veterans blog and the travel blog. It's far from finished, and I probably won't get it finished for a couple of months.
I'll probably spend 5 minutes a day for the next couple of months finishing the page, and it will then just sit there for a couple of years collecting a few pennies of adsense revenue. But, that's not really the point. The point is that with very little effort I've created related links between two unrelated blogs that I will be investing some effort in.
Search engines are important. Search engine visitors tend to be looking for information, not entertainment, and they're more likely to click on ads than your regular blog readers.