Distribute copies of a poem--either Shel Silverstein's poem "Sick" or a similar work--to the students. Have them read the poem out loud. Once that task is completed, ask the students to identify the couplets in the poem. Ask students to share what they have found, and what they have learned from reading the couplet they selected. Start a list on the board of your choices, and soon you will have a list of all the excuses in the poem. Explain how couplets are used to convey a point.
You could use Shel Silverstein's poem "Sick" or another poem with a similar pattern for this activity as well. Once the class has read through the poem as a group, state that you are going to add on to this poem. Recite one line of a couplet that you have created that fits in with the theme of the poem. Ask who is able to complete the couplet with a rhyming line for you. Write the couplets on the board and continue this pattern until the students understand.
Have students create their own couplet poems. Technically, rhyming couplets should have the same number of syllables in them. For very young students, you could leave out this requirement. Furthermore, you might want to ask them to only write one couplet set. You could ask older students to write a full poem that is made up entirely of rhyming couplets and that meets a certain page or a word count.
Consider introducing couplets to your class on Valentine's Day since you will be able to use the famous "Roses are red, violets are blue" example for the students. Hand out a few poems with blank words, where the students have to fill in the words to make the couplets correct. After that, allow each of them to cut out a red heart from a piece of construction paper and write a Valentine's Day poem on it--with couplets, of course--to a family member.