Engaging students in a speech class will help them to become more confident with their language and will increase their vocabulary. If you are an English or speech teacher, making the classes interesting and fun will greatly improve the students' speaking abilities.
Playing a game of Chinese whispers will show students the importance of being clear and enunciating words properly. Give one student a sentence by whispering it in her ear. Have her whisper it to the next student, and so on. When the sentence has traveled the whole class, have the last person says it out loud. It will have morphed dramatically. Use this to show what can happen if you aren't clear and why proper speech is important.
Ask a student to prepare a presentation for the class on a subject of your choosing. Tell him that you will tell him his time limit for speaking only when it comes time for him to speak. He will have to prepare a speech which could be five minutes or 30 seconds long. This will force the student to think on his feet and will make him more comfortable with ad libbing.
Running a mock trial will enable everyone to practice their speaking ability, since you can give everyone a chance to be a witness or a lawyer. You can either pick a famous trial from history or make one up. You will need to write an extensive back story if the trial is to work and involve something that is controversial. Make the members of the class use proper court language, and have a class jury that votes on the result at the end.
Giving the students scenarios to act out will encourage them to use vocabulary they don't usually use. For example, have one student be a news presenter and another be a celebrity who is in the news. Ask them to improvise an interview for the class. To make it more interesting, tell them they can't use certain words. This will make them comfortable with language and will make them more willing and able to think on their feet.