Here's the clandestine work of a group of environmental artists in the UK. I especially like "Moss Graffitti" and "Secret Worlds". Create a "moss paste" and paint a message or artwork onto a wall or a building - eventually your message will appear and last until the moss colony expands and/or takes over.
OR - Create delicate little interventions in public places for others to discover. Here's a little farm crafted into the side of an old brick wall along a public sidewalk. Imagine being the first to discover such a little landscape in a forgotten crack of a building? If I had created it, it would be hard not to stand watch seeing the reactions of people who discover it. Thanks to Megan for this site! This work reminds me of an urban (albeit more kitschy) version of Andy Goldsworthy - who must endure that most of his natural sculpture of twigs and leaves and rocks and water disappears into the wilds of nature once photographed.
This club goes around and fixes up forgotten landscape areas - with or without permission - in order to beautify a neglected planting bed, discourage littering, and encourage care and community ownership. There are cases of some of their "secret" interventions being noticed by local residents who take over the care and maintenence of the tulips or sunflowers they start, including these kids who caught the guerilla gardeners planting sunflowers, but then decided to get in on the act right alongside. Cool!