From the sound it made when it hit, it sounds like the toy might have been one of those fly-wheel toys. They always make the fly-wheel out of metal.
I was at Costco the other day, and saw boxes of them there. It was tempting, but it wasn't really worth the spike in my blood sugar just to keep maybe one out of ten of the toys. My cats would have enjoyed the influx of cat toys, however; ball up the foil from the egg, drop it in the empty capsule, close the capsule, and instant cat toy. Almost all of which wind up under the couch within minutes. The rest get confiscated after a very early morning game of bounce 'em off the baseboards.
But yeah, I too am curious as to what the surprise was.