Instead of using the same boring G-D-C progression, you can take a Cmaj7, which is basically not using your first finger on a C, and interchange it with C, so you can have a different note for everytime you sing the chorus lines.. Sorta different, but it's not bad.
I figured out something like this, it doesn't sound too bad when you're playing it by yourself, adds a little more variation to it.
E >---------------------------------------------------------- B >---------------------------------------------------------- G >11 12-11-------9-11-9--- 7-9-7----4 ...