A couple of good warm-ups:
Honey Walk: All actors walk in place. The audience calls out different things they must walk through. Snow, ice, (ice caps are melting!) then mud, waste, trash, etc.
Pass the Ball of Trash: All players in a line. Ask the players to pass a mimed ball of trash to others (one ball at a time). The ball becomes heavier, until it weighs a ton, or extremely light, extremely big (and light or heavy) or extremely small (and light or heavy) or very smelly. Actors need to show the ball's characteristics in the way it gets passed. The last person in line crosses the stage to the first player and as they do, the audience can call out a suggestion of what the ball of trash becomes.
Documentary or Slide Show: A leader pretends to be a narrator of a documentary and the actors must act out the documentary the leader describes (leader does a Save the Earth like animals in crisis). They can do it as a slideshow/powerpoint with still picture poses or actually act out what is being narrated.
Stunt Doubles: 2 Actors are acting a scene such as dealing with someone who is doing something bad to the Earth. When it comes time to do a "dangerous" step (such as picking up some litter) they call in their stunt doubles. The easier the "dangerous" task is, the funnier it can be (such as picking up gum wrapper).
Lines from our Pockets: The audience will write lines for the actors to say. Someone will collect the lines and not show them to the actors. The actors will act out a Save the Earth scene and then they must work the lines into the scene.
Here Comes Jill: One actor plays Jill who is off stage. The other two actors are two friends concerned about how wasteful Jill is. The two describe the terrible things Jill does while he/she is off stage. Jill enters and she has to act like she is described.
Eco Inventions (aka props): All the actors are sharing their new Save the Earth products. Give the actors weird things they have never seen before and they must say what they do. The object can do anything except what it really does.
Questions Only: Actors get in two lines (one line is Pro-Earth and the other is Anti-Environment). They talk in the form of a question. If they make a statement or don't use a question, then they must go to the back of the line or the audience can shout DIE and they must do a dramatic death scene. The line with the most left at the end wins.
Stand Sit Slump: Three scientists are testing the environment for contamination. One must be standing, one must be sitting and one must slump down on the ground. After each actor says one thing, the last actor to speak must pick a new position (stand) and the others must adjust and pick the two remaining positions (sit and slump). This can continue until a high moment of humor or until one actor messes up.
Melodrama: Do an old fashioned melodrama, but with a twist. The twists will be based on suggestions from the audience. Have three characters: a damsel in distress, a hero, and a villain. Audience: will Boo at Villain, Cheer for Hero, Ahhh for Damsel. Audience will suggest... Damsel: something strange to raise on a farm (wind or solar farm), Villain: a weird form of torture, Hero: an odd weapon someone might use to stop a villain.
Gibberish: A group of actors (group #1) are aliens from another world and act out a scene speaking in a weird alien language. They act out a simple activity like picking up trash or recycling. The other group of actors (group #2) act out the same scene and translate what was said in English.
Variation: have actors translate what was said by the aliens as they act
Variation 2: have a actor play a superhero alien who speaks gibberish and have the other actors try to figure out what he/she is saying in a scene where an alien lands in their backyard Pick your Eco Hero (The Good, the Bad, and the Ugly): You must pick a hero to join your Save the Earth Club. 3 players (superheroes) form a line upstage. The audience provides Save the Earth questions or problems for which they need advice. Actor #1 always provide good advice, actors #2 always gives bad advice , and actors #3 gives really bad advice. Good advice should be good, bad should be opposite of the good and ugly should be an even worse version of bad.
Action Figures: 2-4 actors are the litter patrol. They have a contest to see who can pick up the most trash but none of them can move on their own. They can only speak. 2 audience members must move them in the race to pick up the most trash.
Or check out some Save the Earth themed scripts http://www.freedrama.net/earth.html