Or a horse, of course. The bike has to be rideable, and needs to return to normal afterwards.
Most years my city has what they call a "carnivelo", a family ride in which bikes, riders, trailers, passengers etc are encouraged to be in fancy dress and ride a car-free route. There are photos online and videos of previous years on YouTube. Some years have more fancy dress than others.
This year the theme is "enchanted forest" so I thought a unicorn with 2 riders (my daughter fits better on her bike seat than in the trailer) would be suitable. We would of course be in fancy dresss ourselves. A "horse" could also pull a carriage based on the frame of the trailer with a new cover.
While the event itself is traffic-free, we'd have to get there somehow, which would presumably involve riding in traffic.