Keep in mind that nothing is ever wasted in the universe. Sooner or later, probably much, much later, they will decompose and return back to their basic elements. If you don't want to wait eons, there must be a million uses for them. You could probably build a house if you had a real long time, or fill them up w/ concrete/plaster/clay and use the plugs for something dandy. As mentioned, they make classic stash containers.
I look forward to reading people's ideas, and thanks for a great question. They're just so small, it's gonna be time consuming to make anything big, but if someone has the time, tons of things you could do. I'm curious where creenus is too because I sold my place the other day and am off to live in the Land of Enchantment myself. New Mexico skies are incredible.