Ask the guests to send you baby photos of themselves before the day of the shower. Then everyone can guess who the kid with the cowlick grew into. For added difficulty, include some celebrity baby pictures in the mix.
Baby shower idea: Guess the baby's birthday! Great way to replace a guest book. You could use stickers of a pacifier and/or a rattle. Each person placed the sticker on the date they predicted and signed their name. Perfect to put in a scrapbook, too!