Hey!
There's so many things you could do!
But one of my favourites is the Marble effect is great for mixing colours.
ehow.com/how_5524749_create-nail-design-using-water.html
that link shows you how to do it. it looks so cool 
Also newspaper nails might be nice, but I don't know whether that's prom-y 
P.S. You're dress is gorgeous!