my daughter used to make up songs all the time (she still sings a lot but now it’s usually something she’s heard before) and that’s when i first discovered songsmith from microsoft research.  this is a program that can create a musical soundtrack to whatever you sing.  it’s very fun and there’s a trial available on the website (as well as some amusing videos of using songsmith.  office labs also has a fun game for office called ribbon hero.  you play for points and learn about office along the way.