After reaching level 20, a new cool device Generator is already available for you!
Generator allows you to create new trees and stones on the Big Land or the Archipelago. Please pay attention that you can buy it on the last tab of the Market only while you stay on the Big Land or the Archipelago.
After the purchase, Generator remains with you forever!
Also it depends on you what kind of trees and stones will grow on your island. There are plenty of options!
Now your zombie-workers will definitely have no time to get bored!