This paper shows how strategic menu manipulation in a university canteen achieved a 30.7% reduction in carbon footprint without anyone noticing the nudge. It's such a simple idea that could be implemented in universities, schools, hospital and prison cafeterias etc. It seems harmless and the gains seem disproportionately large when we look at the relative simplicity of carrying this out. The impact could be substantial for animal welfare (cutting down on the consumption of animal-based foods) and climate change.

I don't know what would be needed to get measures like this to be implemented more widely, but it seems promising.