ecoLOGIC Poster Brief [part 1]

I got a brief the other day that is basically aimed at promoting; saving the earth. My first thoughts about it was, great, this should be pretty straight forward to research and generate ideas from.
So, I got right onto having a little mind-mapping session on my, freshly bought, layout pad. It went pretty well the mind-mapping as I got a fairly strong idea to start off with, not that it mattered at this stage, since I now needed to go and do some research to fill the empty gaps of knowledge I was missing out on. The research proved useful, as always. I now had some more ideas, some good, or so I think, and some bad, pretty standard at this point.
One of my ideas involved using the whole 350 x 500mm page covered with pure typography and some sort of colour schemed imagery filling the typeface, leaving the background white. Only problem was what typeface to use? I narrowed it down slightly by opting to use a Sans-Serif because the poster is about present and future issues and by using Serif typeface, I think I could be misleading people unintentionally...