Monocropping field corn to feed cattle has always stood out to me as a big one. Monocrops are inherently vulnerable, beef is a luxury good not a necessity, and the total input for a pound of beef is extremely high compared to most other food items. I haven't personally done the math, but I understand beef production enjoys significant subsidies at multiple steps.

Another topic, a common desire among ecologists is to incentivize more natural habitat at the margins to support native plants, insects, and birds. It's fairly simple, easy to measure the cost & implementation, and the goal would be it would ultimately be better for everyone, with healthy predator populations and so forth.