Here is an OOH campaign for the Artis, Royal Amsterdam Zoo:
Billboards and bus shelters were created with removable stickers of baby animals. Passersby were encouraged to remove the stickers and affix them around town with copy stating:
(this is the one for the baby elephant)
"Expected in Artis: baby elephant (true size)
Babyboom in the city. Help us spread the good news by putting these stickers all over town.