The premium currency in the game is gold coins and they will suddenly appear on the walkways of your Sky Zoo when you earn them. Make sure you keep an eye out for them and when you see them you go and collect them by tapping on them.
Double Gold Coins
Whenever you complete a mission or a set of missions you will earn 50 gold coins apiece for them. If you manage to complete 100 gold coins worth of missions in one run an offer will appear on screen asking if you want to double your gold coins in exchange for watching a free advertisement video.