You could have your girls hold up a sign that says "daddy will you marry mommy?"
Some thing I saw on Laguna Beach Get some thing, they used a kiddy pool, put some fish in it, Write by it, or like you can get a fish tank put a sign on it that says, "Of all the fish in sea Will you marry me?" That's cute.
Get a Build A Bear, if you have one near you. You can put your own voice Saying "Will you mary me?" Dress it in a brides grown, that they have there too.
Ok that's all I can think of right now. Good Luck!!!!
http://tickers.TickerFactory.com/ezt/d/4;10716;6/st/20080622/e/My+Wedding/k/ca48/event.png