We had a "Onesie Decorating Station" at my baby shower; it was a Do-It-Yourself station where my guests got to paint onesies and talk with each other. My shower was held at the house of one of my mom's friends, and we used her kitchen table as a painting station where my guests painted plain white onesies for my baby-to-be. We had various stencils (shaped like animals, shapes, etc.), various paint colors, and plenty of paint brushes. It was really cute, and touching, to see my friends and family personalize a onesie just for my baby.
One of the main reasons that loved the onesie station at my baby shower is because it allowed the baby shower to have a more interactive atmosphere - as opposed to being separated into groups at tables like most of the baby/bridal showers I have had and attended. The onesie station gave people a reason to get up from their seats and mingle, and I actually got to have conversations and laugh with each of my guests as they rotated between the snacks and drinks to the onesie decorating table. I think my guests enjoyed it because they also got to talk to one another.
I highly recommend this. Just make sure that you have a good place to hang the onesies to dry, especially if you want your guests to be able to paint both sides of the onesie, as painting cute stuff on the butt part of the baby onesie makes for a really cute outfit for baby (every baby outfit is cuter when it has cute stuff on the butt, duh).