One year my friend Mark suggested that instead of Happy Birthday they sing Isn't She Lovely to me. I love Mark.
A short list of songs I would consider it very funny to make people sing:
Pretty Fly for a White Guy
Rock Me Amadeus
The Gummy Bears Theme Song
As it actually is my birthday please pick something you think is amusing and sing/hum it around your office until it's stuck in all your co workers heads. That would totally make my day. If you don't work in an office family members and/or friends will do just as well.