Worldwide Tree Planting
This is a big one if we find many voluntary Planet Guerillas. It is about planting trees at any spot that is not used for anything else. There is massive space along every highway, every road, every track across the fields, parking lots and others. Everybody can identify spots and discuss with the relevant authorities whether they would allow and support (and maybe even finance) such planting.
If they agree but do not have the cash, collect some seeds, plant them in a pot at home and get them out as soon as they are big enough. You would need to protect them with three poles around them to protect them from mowing. Acorn, beechnut, apple, cherry, hazelnut or whatever you have available. Everything helps.