The game starts with the protagonist moving to Ranch Island to start their life as a farmer. The goal of the game is to collect Sun Stones and bring back islands that have sunken to the ocean floor. There are 100 Sun Stones that will restore 10 islands. The different islands have different features, such as Mushroom Island grows wild mushrooms.
Sunshine Island is very similar to Harvest Moon: Island of Happiness. It features the same characters and designs but a different scenario, some new characters, and some other additions. The game includes two new love interests: Will and Lily.
In terms of gameplay, Sunshine Islands doesn't require touch-screen controls like Island of Happiness did. The game dials back the Heart Events to 4 (instead of 96 different versions like Island of Happiness), however in their place are 30 new events involving various NPC characters. There are more farm animals to obtain, pets come in two colors, and you can befriend wild animals. On top of that there are more fish to catch and more recipes to cook.
This game provides examples of:
- Continuity Cameo: The Friends of Mineral Town incarnations of Cliff, Doctor (named "Trent"), Karen, and Popuri appear with new clothes. You can tell its the FOMT versions because Karen has her blonde bangs (unlike her DS counterpart).
- Crystal Dragon Jesus: The Harvest Goddess is a pagan goddess that is worshipped in an awfully Catholic-esque manner. There are also apparently different sects of Harvest Goddess worship if an event between Nathan and Cliff is to be believed.
- Cutting Off the Branches: Assuming these are the BTN(FG)/(M)FOMT incarnations of the characters, Rick and Karen being married means that either Pete/Karen or Claire/Rick aren't canon in those games.
- Gotta Catch 'Em All: The 100 Sunstones. Turning in varying amounts of Sunstones to Agete will allow you to raise new islands.
- Green Rocks: Wonderfuls in Sunshine Islands are attached to your tools to increase their abilities, i.e. the range of your hoe.
- Mission-Pack Sequel: It shares roughly the same mechanics and same character pool as Island of Happiness.
- Mistaken Identity: Karen and Popuri mistake Mary for Sabrina. This also lampshades Sabrina being a Mary expy.
- Mythology Gag: Taro and Chelsea dress like the former protagonists Pete and Sara.
- Updated Re-release: Sunshine Islands is one to Island of Happiness.