Consider observing water quality. I work for Foldscope instruments and we have a low-cost microscope that you could distribute to every student. The Foldscope is a low-cost microscope made of paper and a small glass bead for a lens. It costs less than a dollar to manufacture and we sell them in packages of 20 for $25 (that's just $1.75 per microscope) They have 140x magnification and 2micron resolution, so very much like a microscope you would find in a science lab. And because they are so inexpensive, every student can have their own! See our website for more info.
You can take water samples from a local pond and observe under a Foldscope. Often times students will see numerous organisms in the water. You can try the same thing with tap water and see if they see anything.