Two teens girls need to write a duet and record but what don't know what to write about?

My neighbor and friend is a music producer and has a small studio in his house, and since he knows I can play guitar, and that my friend is a singer, he asked us if we would WRITE AND RECORD a duet in his studio that he will put together for his resume. We are so excited! Its a fun little project. Nothing special (: But we will both be singing a duet and we don't know what kind of duet two teenage girls could sing. We are both 15 years old. Do you have any topics or ideas? (Yes we are best friends, but I think a "youre my best friend" duet could turn out to be way to cheesy) (: