As noted before, I love to change song lyrics. I like to make them dirty or silly/outrageous. I am pretty quick, but recently some songs have been lobbing up softballs for me. My two personal faves:
"tick tock, ride my c-ck, til i scream out mmmm stop" - All other changes are centered on sex acts involving the male member.
"1-2-3 get an STD, goin' down on britney, it's gonna hurt when you pee" - Lots of changes you can insert here because of the "ee" sound.
The possibilities for change almost make the songs downloadable.