During testing season we adopted an upper-grade classroom and retooled songs like the "Wheels on the Bus" to motivate them to score well on their big tests. We used a bunch of different songs and made the lines for syllables, trying to come up with new lyrics that fit.
I like to do a kind of year-end exit ticket. This year I tried something different. I had the kids break up into groups and write a song detailing their experiences in First Grade. They chose the song themselves and wrote all the lyrics. Now, they didn't come out exactly like I had envisioned, but I do think they did a great job. Of course, there's a Let It Go parody!