In order to introduce students to the concept of habitats and ecosystems it is often interesting to look at the microhabitats that can be observed only when you look very closely. Plankton which live in ponds, lakes, and other bodies of water form the foundation of the entire aquatic food chain. Students are fascinated by the complexity and diversity of life that can be observed in a single drop of water. If you need to collect plankton yourself, it is best to observe the samples that day or no longer than 1-2 days later. Many plankton are fragile and tend not to survive long outside their natural habitat, particularly at the population density obtained after using a plankton net. However, some plankton can be maintained in the classroom for several days or even weeks under the right conditions - adequate dissolved oxygen and access to a food source. In fact, an interesting extension would be to monitor the types of organisms observed by the class over time as species die out and other increase in numbers.
Experience with microscopes allows students to jump into the classroom portion of the activity more quickly but is not necessary.