From the Super Mario Wiki
Sunshine Isles is a multiplayer level and an extra level added exclusively to Super Mario 64 DS, located in a painting inside the Rec Room. In regular mode, it has five Silver Stars that the player must collect for a Power Star. In multiplayer mode it has Power Stars players must collect. The level is known for having a remix of the music from Delfino Plaza in the game Super Mario Sunshine, although using the same instruments as the rest of the game. As the name implies, it is a group of islands, which are connected by shallow water. It is unknown if they are actually a part of Isle Delfino. However, there are many big islands seen in the background, of which one could be Isle Delfino.
The player must collect the five Silver Stars to make the Power Star appear. Silver stars are scattered throughout the isle, including inside a cavern or on the top of trees. A Klepto is also holding a Silver Star and the player must intercept the Klepto in order to gain the Silver Star. If the player touches an enemy, the player will lose one Silver Star that will jump around, just like how they do before the plumber obtains one.