Find the most popular queries, built content pages around it and add ads there And you could try some alternatives to AdSense like: Ezoic, MediaVine, or Adthrive
Another online possibility: throw up some Amazon (or similar) affiliate links to products and services, maybe even based on what people type (if that's accessible).
And next to them "opt out of the ads" link that goes to a donations website, to get fancy. Helps clear the conscience about having the ads in the first place.