Something I have always wanted to do is bundle up wild edible plants and scatter their seeds across the landscape. I already do this on a limited basis on my walks - I'll stick red oak acorns, white pine cones, and white cedar seeds in my pocket and scatter them in needy places.
Other folks take this concept much further:
NPR: Environmentalists Adopt New Weapon: Seed Balls
CSMonitor: Guerrilla gardening takes root in Los Angeles
BBC: Guerrilla gardeners wage turf war
