You can improve and expand Mystic Bay by adding decorations around the island. You can get decorations in exchange for gems—just tap the house icon in the lower-left corner of the main island screen to view available decorations and go shopping!

Placing decorations will net you Island Points, which improve your Island Rank. Sometimes, you need to progress in the game first and reach a certain Island Rank before you can purchase certain decorations. To learn more about Island Rank, tap here!