I echo the sponge painting idea. I also use them to clean up white glue on mdf buildings. Just wet them a bit and slowly work over any excess glue. It will reconstitute and wipe off. And then you just throw them away, and haved used the "nice" kitchen sponges.
They can also be used for trees. Cut them into clumps and glue/impale them onto your tree armatures. Whether you then just dry brush them in greens, or glue on flock/foliage is up to you. Perhaps you can make coffee grinder foliage to glue to your trees!