Quick Play Mode:
Beat the Adventure mode once and Quick Play Mode will be unlocked.
Finish Adventure Mode once, play Crazy Dave's Challenge when he picks 3 of your seeds. Reach stage 4-10 and the Yeti Zombie will appear.
Dig all the way to China:
Scroll down like a crazy person on the awards page! After a while you'll have dug so far that chinese zombies start to appear.