What song should I sing for this festival?

It's a solo and ensemble festival. I'm doing a flute duet as well. However, there are some restrictions for what I can and cannot sing. It cannot be Broadway, jazz, pop, etc. it must be a clical or folk song type of thing. I am a mezzo-soprano with a three octave range. I prefer not to do an aria, although I suppose that is an option. Any suggestions? I'm at a dead end.