I really like the idea of reactionary art.  The first thing that comes to mind with having an environmental emphasis is something that earth first! did to some dam that they were protesting outside of.  Two people went up to the top and center of the dam and dropped a black sheet, that when it reached the bottom it looked as though it was a crack going down the center.

Something that maybe we could do is drape a sheet across trees, or obstructing the view of the lake, and have different buildings be the only thing that they can see.  Or maybe painting the windows black on a sunny day so that people will, eat, talk, conduct class, and so forth, outside.  

As far as advertising goes, i think that we would get a better turn out with anticipatory advertisements.  The kind of advertisement that only gives people a glimpse of what's to come.  Like, a flyer that just says," Are you going to be there...(date and time).