I presume this question has already received a response since it is already six months old.
First of all, the word is environment, and as a teacher, you should clarify the correct spelling: Not enviroment, not envirement, not enviorment, not evironment, but environment. If you want one lesson that is introductory, you may ask what the children understand by the term, elicit and discuss some responses; otherwise, narrow down the topic. The possibilities are clean air and water, deforestation, fossils fuels and alternatives, conservation of resources, garbage and recycling, acid rain, global warming, and sustainable agriculture.
For a single lesson, you might select a few of the topics, ask what the children know about them, and explain what the issues are. Bring a recent newspaper or magazine article on one of the issues, have the children take turns reading and discuss the issue. Alternatively, you can make a reading guide in which you ask questions whose answers can be found directly in the article, answers that are inferred from the statements, and some questions asking the reader's opinion. Make sure to provide a "safe" atmosphere in which opinions are respected as long as they are supported; insist that the children say more than, "Yes" or "No." Older children can engage in a formal debate on an issue. Let them know that there are no clear answers to some of the problems: there are valid issues of job creation and security, and preserving the environment for ourselves and future generations; there are conflicting interests among groups of people and people in different geographic locations; there are economic advantages, and problems of chronic illness.
Check out the materials available for labs in science materials catalogues. There are many interesting activities that can be planned with these materials, whether as part of a long-term curriculum, or as stand-alone labs. By involving the children in discussion and providing plenty of hands-on opportunities, you provide an environment (no pun intended) in which they can be active and engaged, and will be likely to act in a positive manner. For a long-term assignment, you can give different topics to each pupil or to groups of children; they can then present their findings to the rest of the class when their work is complete. It can be an essay, which is summarized, or a visual project with a presentation boards. Take them outside when you can; children are always glad to get out of the four walls of the classroom. They can make a solar oven, or observe the effects of solar radiation on yeast cultures.