Buy a variety of large, fake plants. Real plants can be used but choose plants that will last and that are very green. Real palm leaves are perfect to use. Arrange the plants all around the room. The effect should make the classroom look it has been overgrown.
Have students make "vines." Cut strips of green construction paper. Glue or staple one strip to create a circle. Put the next strip through the circle and glue or staple it to create another circle. Continue until you have a long chain, then hang them all around to look like vines.
Add animals. Arrange stuffed animals of creatures that could be found in a rainforest among the leaves around the room. The more life-sized they are, the better the effect. Gorillas, frogs, butterflies, elephants, monkeys and snakes are all good to use.
Add rain sound effects. Buy a rain soundtrack online or at a nature store. Or, bring in a rainmaker, a musical instrument that sounds like rain. Rainmakers, also called rainsticks, are available in music stores. Children can take turns making it "rain" in the rainforest.