Hi Michael, SAPS (Science and Plants for Scools) suggests algae wrapped in jelly balls and they have lots of online resources for their use (http://www.saps.org.uk/secondary/teaching-resources/235). I haven't tried this method myself but I've heard that it works really well. You might have to contact SAPS directly for advice about how to obtain the algae though... saps@hermes.cam.ac.uk. Good luck!