Classroom activities are sometimes a difficult process of trial and error. You often don't know what's going to work until you've already tried it. However, if you pay attention to your students needs you should be able to find some activities that they find fun. For example, if you are teaching a beginner ESL class, try having the students in your class introduce themselves to each other in pairs. Then, have each partner announce the details she remembers to the class. This will force your students to change the form of "be" from the first person to the second person.
Several websites such as Iteslj.org, A4esl.org and ESLgames.org have fun games you can use to teach forms of the verb be. Prepare by writing a bunch of nouns and forms of the word "be" on scraps of paper. Lay them out on a table and divide your class into two teams. Students compete to match a noun with a form of the word be. When they are correct they get a point.
If you're teaching young children, songs are a fun way to teach just about anything. Associating words with a rhythm can be a powerful method of memorizing just about anything. Pick an easy song that your students will already be familiar with and substitute basic sentences that reflect the correct usage of the verb "be." Make the song catchy and repeat it often; this will help your students remember the basic forms.
It's never enough to merely teach your students; you'll also have to reinforce what they have learned. Older students may find it more interesting to simply have conversations. Try listening to them talk and correcting them when they make mistakes. If you are teaching a particular form of the verb "be", tell your students to focus on using this in their speech.
Try reading some simple books to your students and see if they can identify the form of "to be" in every sentence. Stick with a simple children's book that features simple grammar. Tell them to put up their hands when they recognize a form of the verb. You may also wish to photocopy some pages and have them underline the correct verbs. Try and get them to write some examples of their own. This activity can be scaled to the level of your class.