The Sea Slide contains several islands and caves, and a big tree. Shells can be picked up for quicker movement through water.
The Middle Island is half water, half land. Penguru looks from there to see what is going on on the sea slide.
The Shrinking Planet appears when you fed the Hungry Luma enough Star Bits. Unlike Hurry Scurry Galaxy, here you have to collect Musical Notes around the whole shrinking planet, which makes it difficult. For a tip, the player should wear a bee suit (turn into a bee) before launching off with the planet, that way on the shrinking planet you can fly across gaps when platforms are shrunk.