Going to school is often dreaded by students. It's hardly perceived as fun. However, one way to make lessons more enjoyable is by injecting some humor into them. For example, one way for teachers to make teaching plural rules more fun is having students write a story where they invent a character and then use that character's actions to demonstrate things like plural nouns. At the end of the activity, students can read their stories to the class.
Another fun way to help students learn plural rules is to play a game. One example is to divide students into groups or teams. Then the teacher will read a sentence and the students will discuss among their groups whether the noun should be singular or plural. The teacher could also give students a noun in its singular form and ask the students to make it plural. For example, "box" would become "boxes." At the end of the game, the team that gets the most words correct receives extra credit or a prize.
Singing is another fun way to teach grammar. It's also an effective way for students to retain information, especially if it's with a catchy, simple song. However, you don't need to write a new song to learn plural rules, you can take an existing well-known song and emphasize the parts that use plural nouns as you sing. Take a common children's song, such as, "Mary Had a Little Lamb," and rewrite it so all the singular nouns are plural.
One way to break the monotony of the classroom is to take the class outside. It's less likely that students will become restless or inattentive with an outdoor activity, which helps keep them more engaged. There's no limit to the ways to teach plural rules in the outdoors, either. For example, teachers might ask students to take a notebook outdoors with them and write five sentences using plural nouns based on what they observe. Once back in the classroom, each student can read their sentences and the rest of the class can be the judge of whether their use of plural grammar was correct.