One way to get more coins and gems is to use the trade boat and visit neighboring islands to sell off extra crops and crafts. The more coins and gems you have, the more you can do on your island. The premium currency in the game is gems and the best way to earn these is by completing quests, Daily Requests or Achievements. Try to save your gems, they are important and shouldn't be wasted on speeding up harvesting and crafting actions. There is always the option of In-App purchases using real money if you want coins and gems immediately.
Starter Pack 01 $1.99
Box of Gems $4.99
Trunk of Gems $19.99
Suitcase of Gems $9.99
Stack of Gems $2.99
MEGA Pack $9.99
Bag of Coins $9.99
POWER Pack $2.99
Pouch of Coins $2.99
Value Pack $0.99