This is not a lesson plan but an interesting way to motivate students to speak French in immersion classes.
Based on the Fall theme(or other themes), draw and cut out a large tree with as many large branches as you have students (each student "owns" a branch). Staple it to a large bulletin board. Students start off with ten leaves which they staples to their branch. If they speak French during the day they earn more leaves. If they speak English when it's not English period, a leaf falls from their branch. If it is Fall the tree can look quite colourful. If there are very few leaves on the branches...the tree looks bare. The teacher can add a nest and birds to various branches as a sign of extra good work or effort. If the tree is very "healthy" for a long period of time, a party or other incentives could be given to the class. Of course, there are many variations to this idea. Have fun!