Collect a cushion of moss from a wall to observe through a microscope (if you have a video camera attachment, even better). Avoid over-collecting - one cushion will provide more than enough material for a class. Soak the moss in water, then tease it apart with mounted needles. Best results are achieved when each microscope user has only a small piece. Place the moss on a cavity-slide with a few drops of the water it was soaked in, put on a coverslip and view under low power. Water bears are about 0.5mm long, so should be clearly visible, along with other fascinating microscopic creatures.
* Edinburgh Tardigrade Project: http:zeldia.cap.ed.ac.uktardigrades_org.html
* Tardigrades, an account by Phil Greaves, a water-bear expert: www.nhm.ac.ukhosted_sites queketttardi.htm