Earn coins by purchasing plants, trees and animals. After you feed/water them into maturity, you'll be able to harvest them for coins, XP and sometimes energy. You get occasional crystals from leveling up and completing tasks, but the fastest way is to go to tapjoy.com. They give you crystals for completing offers. Many are as simple as downloading a free app and running it. I've earned hundreds this way and haven't spent a dime.