Bring in a top-40 song that all the kids will know/like, and have them re-write the lyrics to fit whatever concept you're covering at the time. They'll have tons of fun despite the challenge, and then they can make fun of each other while they perform their new song (and review material without even knowing it).
Example: (to the beat/tune of Nelly's "It's Gettin' Hot in Herrre") In science class, you could sing "It's gettin' rainy in herrrre, barometric pressure shows..." Cheesy, yes, but it's just an idea! =)