The first letter of all the levels leading up to the final level spell out "Crowned":
• Cookie Country
• Raisin Ruins
• Onion Ocean
• White Wafers
• Nutty Noon
• Egg Engines
• Dangerous Dinner

This refers to the final boss, Spoiler:Magolor, as he uses the master crown in the final battle.

Kirby's Return to Dream Land underwent an 11-year development cycle, perhaps one of the longest in the series. It started right after the release of Kirby 64: The Crystal Shards, designed for the GameCube. This project featured 2.5D visuals (similar to Kirby 64) and co-op multiplayer, was tentatively titled "Kirby GCN", and was one of 3 canceled projects.

The second one was in a fully 3D environment, yet had not achieved the quality Nintendo had expected; the Kirby series would not revisit this approach for a full game until Kirby and the Forgotten Land in 2022. The third one was once again a 2D side-scroller with pop-up book visuals. These projects were eventually recognized as failures due to how most of the focus was on multiplayer instead of the game itself.

In modern iterations of Fighter Kirby's moveset, the charged attack Instant Mega Blast uses the same key combination as Ryu's Hadouken from Street Fighter, likely meant as a tribute to the series.

There were originally going to be more Super Abilities. A Super Spark and Super Cutter were planned, but were cut for two reasons. The Super Spark data is completely overwritten with Ultra Sword files and the Super Cutter was cut due to it being to similar to Ultra Sword. The model and icon for the Super Cutter still remain on the game's disc.

An unused alternate design for the enemy Walf can be seen in the game's files. The early design is missing the blue cap, has a brighter shell color, a simpler eye, and no alternate facial expression textures.

Some of Kirby's rock transformations reference Kirby characters from past games. An example of this is Marx, who also makes a cameo in the Kirby Master video.

In the final battle, a song called "Halberd" plays, but the Halberd is nowhere to be seen. It is possible that the song is a left over from when Meta Knight wasn't considered to be a playable character.

When Kirby's Return to Dream Land was localized in Europe, the title name was changed to Kirby's Adventure Wii and has the same box art from the Japanese version, rather than the box art from the North American release. This name change was not carried over to the Nintendo Switch version and was released with the original Return to Dream Land name.

Depending on the color the player chooses as Kirby, if they interact with one of the flowers, the flower will turn the same color as the Kirby being played.

There is a unused enemy in the game's data named Hearbell. It was originally going to eat and chew Kirby, much like another enemy in the game, Pacto. It's unknown why this enemy was removed in the final release.

Kirby's Return to Dream Land Deluxe redesigns Wing Kirby's costume, changing the pattern on the headband to a series of straight lines, adding a red jewel on the front (similar to the kind seen on other Copy Ability hats), and replacing the rows of feathers on the back with arrangements that more closely resemble a bird's wings. As the original design was based on a Native American warbonnet, the redesign was likely to avoid cultural appropriation, as real-life warbonnets are regarded as sacred in tribal cultures and consequently can only be worn by those who have earned the right to do so.

Within the data of Kirby's Return to Dream Land is an unused plains theme, labeled KbyW_HEITI1 in the game's code. The song features a leitmotif in its second portion which is also quoted in "Beyond the Hill" and "Perfect Sledding Weather", with a variation also appearing in "Onion Ocean". Curiously, this leitmotif is also featured in the final boss theme in Kirby's Epic Yarn. Given that the leitmotif has no apparent narrative significance, it's possible that KbyW_HEITI1 originated from one of the three cancelled Kirby games for the Nintendo GameCube that eventually evolved into Kirby's Return to Dream Land.