Yuna's wedding dress ( http://www.deadlybrain.org/torrent/games/pics/ffx_wedding1600x1200.jpg
) would work really well, especially for an outdoor wedding. You could even do something with a more traditional skirt and just keep certain elements, like the wings and feathers along the bottom edge. If you work numbers, it might be better for you to buy a dress during a $99 sale and then just add the feathers.
Princess Garnet's corronation grown would also be a beautiful wedding dress. Sadly, I can see it taking lots of money to make.
For your cake, I think some bakeries offer to put you picture on your cake. They just print out the image you give them on an edible rice paper piece. You might be able to find that somewhere around. I know Vons does it.