Offering a piece of chalk to draw with.
Discover/write a secret note.
It reminds me of a similar concept I posted about in 2010, Dead Drops, which allows anonymous sharing via USB flash drives hidden around the city (I wonder if they're still there?). With all of these projects there is always some risk involved - vandalism, inappropriate 'offerings', a virus or two, but especially with creative street art projects, it is the risk factor that gives it some oomph. Street art isn't supposed to shy away; it should confront people who discover it in their environment.
Interested in street art? Browse the Street section of Creative Collision.