We did one once (very minimal decorations) but saw some amazing ones. One girl wore a Dorothy dress and had a Toto in her basket; I can't remember how the bike was decorated. If I remember any others, I'll try to come back and post.
What about a bird's nest for Robyn? You could use a hula hoop and papier-mache to make a nest, and then dress her in all pale blue (Robin's egg blue) and he helmet could have eyes and a beak. That would be super cute!
Or make a tutu for the bike and make it a ballerina bike?
Or make her a castle cutout on the front and dress her as a princess? Or Cinderella's Coach and be Cinderella? You could do a princess/maiden cone hat with streamers on top of her helmet.