Exercising the Larynx is like exercising any other muscle, and so it needs to be properly warmed up. Have your class begin by making a humming sound that gradually raises in volume. When the humming has gained some strength, tell them to release it with a "muh" sound. In addition, or as an alternative, instruct your students to make "mmmm," "ahhhhh," and "ssssss" sounds with strong or exaggerated facial features. You can use other similar simple sounds to create some variety.
Breathing exercises are useful for increasing lung strength and thus increasing projection. One popular breathing activity is to sit down with a straight back. Place one hand on your stomach and the other on your back at the same height. Breathe deeply so your hands move away from each other and let the breath out slowly with no sound. If sound escapes, it's a sign of tension and simply repeat until it can be done quietly. Then breathe in deeply once more and while exhaling, and say the first letter of the alphabet. Repeat the process with all 26 letters, slowly gaining volume as you go.
Bring in a newspaper or the Yellow Pages and instruct your students to choose an advertisement to read dramatically. Focus more on enunciation and placing emphasis on words instead of simply shouting the entire ad. Don't be afraid to let them have a little fun and really exaggerate the tones or read it comically. Try having your student read it a second time, but place the emphases in the wrong places.
Split your group into pairs and have them talk about a holiday or other simple topic. Every 15 or 20 seconds, say "louder" or "softer," so your students have to change the volume of their conversation. This is a fun exercise and can lead to a lot of laughter, as strange phrases end up being spoken loudly. As a supplement, you can continue this exercise by placing the pair about three feet away. Every 15 seconds, tell the pair to move another three feet away from each other until they are on opposite sides of the room. They must continue their conversation, despite the distraction of other voices around them.