Sunshine Isles

Sunshine Isles is a special level in Super Mario 64 DS, being accessed in the Rec Room. It is an archipelago of small islands in shallow water, with several trees scattered around the islands. Notably, this level shares its theme with Delfino Plaza from Super Mario Sunshine.

Geography
There are 6 islands in Sunshine Isles. There are 5 smaller islands, and 1 large island in the center. There are several trees on the islands. There are also 5 Silver Stars scattered throughout the level. The Silver Stars are above trees, in a hidden area behind the large island, held by Klepto, on top of the large island, and on top of a regular island. These Silver Stars unlock the level's only star. Enemies in this level include a Piranha Plant, Goombas, a Lakitu and Klepto.

Theorizing
It is believed that Sunshine Isles is a reference to Super Mario Sunshine, due to the level technically being a beach level, and Super Mario Sunshine having a common theme of beaches and the ocean. The level theme is also a remix of the Delfino Plaza theme from Super Mario Sunshine, further supporting this theory.

It has also been speculated that, much like other aspects of Super Mario 64 DS, Sunshine Isles was a scrapped level in Super Mario 64 that was added to Super Mario 64 DS either due to direct inspiration from the level being unused, or due to the developers finding it in a personalized copy. However, if this is true, it is not entirely known why Sunshine Isles would have been scrapped, potentially due to the removal of the Rec Room, where Sunshine Isles is accessed in Super Mario 64 DS.

Super Mario 64 Disk Version Connections
An anonymous developer of an early build of Super Mario 64 Disk Version for the Nintendo 64 Disk Drive has claimed that Sunshine Isles was a scrapped level for Super Mario 64DD around the time Super Mario 64 DS was released. It is believed that this version of Sunshine Isles was not a reference to Super Mario Sunshine, and was instead simply a reimplemented level that was scrapped from the original version of Super Mario 64. Notably, this version of Super Mario 64DD would have utilized the Disk Drive's Internal Clock, meaning that the supposed Moonlight Isles level may have also appeared, though Moonlight Isles was not mentioned by the developer, making this theory hard to confirm.

Super Mario 64 Manifestation
In some personalized copies, Sunshine Isles has been reported to be found in the original Super Mario 64. It usually has a red coin star in place of a Silver Star star, though if Silver Stars are also in that copy, there may be a star obtained by collecting Silver Stars, either as an addition to the red coin star or in place of it. A third star reported in this version of Sunshine Isles is collected by obtaining 5 secrets, likely due to their similarities to Silver Stars.

Moonlight Isles
Several have claimed to remember a night-time equivalent of Sunshine Isles named "Moonlight Isles" found somewhere in either Super Mario 64 DS or the original Super Mario 64, most commonly in the Third Floor due to it's night-time theme, or replacing Sunshine Isles in Super Mario 64 if it is night-time through the Internal Clock. The level has been reported to be either a night-time version of Sunshine Isles or an entirely different level, and the star or stars in this level are inconsistent between reports.

Level Expansion
Sunshine Isles can be reportedly expanded into an entire full course when encountered. This typically contains other common missions, such as defeating a group of enemies or collecting 8 red coins. Notably, several references to Super Mario Sunshine can also be found in this anomalous version, such as the Piantas appearing or Wiggler being fought, reminiscent of one of Super Mario Sunshine's boss fights. Said Piantas can also give the player stars similarly to Toads, which is another common mission. The geography is also expanded, with more islands and often a town to fit the Pianta NPCs. Notably, a unique NPC known as "Locks" can be found in Sunshine Isles that appears to be an unused human model from Super Mario Sunshine. Why they manifest here is unknown.