I plan on adding Zombies and aliens in "hidden" waves in the game. My current plan is to have a hidden item that if you shoot or collect it will unlock the new level to the wave list. 
But should these items be hidden or shown? What is most fun? Having easter eggs that are exclusive or something fun that everyone gets to play? I can see both sides. 
Any thoughts? 

Saucer Deer

Zombie Deer

Perhaps there can be a secondary hidden item after you get the wave. like a new starter weapon or a Gadget you get if you clear the wave?