I can give you the themes of past derby days and greek sings. all of them did generally well in the end.
A candy theme: all the songs pertain to candy and other sweets
A "crazy" theme: everyone dresses like they are in a psych ward and do songs like "insane in the membrane" and "freak out." Phi sig did that last year... awesome.
A jungle theme: guns and roses, lion sleeps tonight... you get the idea.
a rescue theme: everyone dresses like firefighters, drs, lifeguards, etc. We did that last year and we brought back the Baywatch themesong which was stuck on our heads for days.
A party theme: all songs pertaining to parties
biker chicks/heavy metal: every time i see this done, it's done well. gamma phi did something like this for derby days and kinda ended with a downhome american feeling.
I'd tell ya what our theme was for this year's greek sing, but it's a secret until friday