My idea: Create a slogan encouraging those who use public land (especially National Parks) to remove one piece of trash from the landscape during their visit. Put it on the brochure for that park. Make a sign or two. If you add it to the brochure, it won't cost a thing to produce.
It came to me as I spent last weekend in Yosemite. While I was hiking, I came upon a discarded Vitamin Water bottle that someone had thrown away on the side of the trail. I picked it up. Then I came upon a candy wrapper. I picked it up (and put it in the bottle). At the end of the hike, I had a bottle full of trash. I ended up turning it into a game and actually made it enjoyable.