If you live in a med sized city you likely have the chance to be the change you want to see if you're willing to be in somewhat questionable legal territory. I'm part of a local arborist group that has the goal of getting my and the surrounding neighborhoods up to 40%+ tree cover. It started with two guys who planted hundreds of (now) 60-100ft native trees under the city's nose in alleyways, hellstrips, the yards of rental properties, newly sold houses. By the time they got found out neither the city nor the residents could really be mad at the positive vandalism and so they went legit and now the city just pays our little group to keep doing it.

It's so fun going door to door and being like, "would you like to accept our lord and savior the silver maple into your yard?" We recruited an ecologist recently so we're expanding into native gardens and grasses.