I put a giraffe plushie on the top! It hides the wine bottle completely so I think they will really be surprised when they unpack their gift.
I also bought a pack of plastic marigolds at the dollar store. I glued 3 on the ribbon (to hide where I glued the ribbon ends together), the other 3 I just placed on the cake.
Source:
http://www.squidoo.com/diaper-cake-tutorial