pink for a girl, blue for a boy; have the MD seal the gender in an envelope, then take it to a balloon shop to put colored balloons in a box to open together later or to a bakery to have them bake you a colored cake.
This is how I plan on asking the maid of honor & bridesmaids. Really cute idea. 5 Creative ways to ask: Will you be my bridesmaid? This photo is a DIY balloon in a box that you have to pop to find a secret message inside!