Inflate yellow balloons with helium and tie with curly yellow ribbon. Add a Rubber Ducky candle to end of the ribbon to add as a weight to hold balloon from flying away. Perhaps have one for each guest to take home.
Stretch a clothesline across the room, using clothespins to display tiny baby clothes, shoes and cards -- especially fabulous if you have actual clothes from the mommy and daddy when they were babies...
Use diapers for place mats and pacificers as napkin rings.
Float extra large rubber ducks (found where pool supplies are sold) in the pool.