Growing crops
Crops are a major component in creating an economy for your city, providing both materials for goods in the shops and ingredients for crafting.  Most of the crops unlock as an achievement for leveling up.  The others are tied to quests throughout the game.  The cost of planting the crops is offset by the profit from harvest so don't be afraid to drop the more expensive crops in the ground.  The only concern is if you have enough coins to pay for the initial planting and enough energy to bring in the harvest.  Here is a list of the crops currently available in the game (click on the crop to see details):
onion broccoli
flax tomatoes
bamboo prickly pear
fig sunflower
beetroot pumpkin
rice strawberries
corn poppy seed
wheat cucumbers
giga-carrot pineapple
chili pepper garlic
red beans blueberry
grapes potatoes
radish cabbage
lettuce spinach
peas asparagus