Find instructions for a two-tier diaper cake that works for either gender at the Make Baby Stuff website. You need about 16 diapers, rubber bands, ribbon and a few toys.
Roll up 22 diapers into a tight tube, and place a clear rubber band on each end of the diaper tube for a uniform roll. Collect six diapers in a circle for the top tier of the cake, and wrap a large rubber band around all of the diapers in the middle so that it can be easily covered with ribbon later. Repeat the same process for the bottom tier of the cake, and then place another layer of diapers around the first circle so you have 16 in all. Wrap a piece of string around the entire bottom tier to hold it together.
Choose a neutral color of ribbon such as green and pink or yellow, and wrap it around the center of both cake tiers, making sure to cover up the rubber band and string. Cut out a round piece of cardboard and spray paint it a neutral color, or use a platter and place the diaper tiers on top of it. Stuff a rattle into the center of the top diaper tier, and place toys, sippy cups or other useful baby items around the cake and tucked in the ribbon.