I am a musician. As a songwriter, I know it is virtually impossible to write a complete song with correct grammar, simply because there are so many other goals that require more focus for song writing such as rhythm and timing of the lyrics. Knowing this, and knowing how much people love music, regardless of whether they are a musician or not, such an idea can be integrated into a lesson plan!
After focusing on the rules of grammar and spelling for several weeks, I think a good assignment that can close a unit on grammar is allowing students to edit their favorite songs. Firstly, I would ask every student to choose a song that is appropriate and free of profanity. Secondly, I would ask every student to identify and circle every grammatical error within their chosen song. Thirdly and finally, I would ask the students to rewrite the song, using correct grammar. For the song rewrite, the lyrics no longer have to rhyme and can even be written in a standard paper format.
Below is a link to a website that I found very amusing, as it points out songs that have the most atrocious grammar. If students have difficulty finding their own songs for this editing project, I would provide suggestions from this site.
20 Songs With Really Bad Grammar