Spikes and Fire
You'll find Corona Mountain is completely different from the rest of the places you've visited on Isle Delfino. It is a huge, lava-filled cavern with danger in all directions. You must get through several obstacles to reach Bowser.
The first obstacle is a series of platforms. Some are ablaze, while others are rigged with spikes. You must use the Hover Nozzle to douse the fiery platforms, and carefully and quickly time crossing over to the spiked platforms. There is absolutely no room for error here; one false step into the fire, spikes, or lava will cost you a life. Some platforms are safe, and one has a 1-Up Mushroom you can grab. At the end of the series of platforms, you can refill your water and ground pound a crate for a 1-Up. However, you'll now be taking a tricky magma river down the cavern, using a row boat.
Crossing the Magma River
Hop into the boat. If you're totally unsure what to do then go to Noki Bay and practice with the boat there since it works the same way but you are forgiven for mistakes. To steer, shoot in the opposite direction to that in which you want to go. You travel down a river with several rocks in the way. If the boat touches anything, you'll sink the boat and lose a life. Steer around rocks by slowly spraying your Spray Nozzle, and if you drift close to one, spray hard at it to push the boat away. There are fountains in the river you can use to refill your water, but don't crash into them, either.
At the end of the river, you'll find blue coins surrounding a big rock platform with a Rocket Nozzle box. If you want to get the coins, be careful and steer the boat and move around on the boat to where you touch them. If not, grab the Rocket Nozzle and get ready, because there's nowhere to go but up.
Into the Clouds
The only way to reach Bowser is to blast up right under the cloud-shaped shadow on the rock platform. You'll end up landing on a cloud of all things. Now, you must blast higher and higher up through a bunch of clouds. Another one up is here, but tricky to get. You'll find the higher up clouds are moving in a circle, and you must time your blastoffs carefully to reach them. Falling can damage your health. The highest clouds are moving in and out, and require good timing to reach.
When you blast up to the top of the clouds, you'll reach a dark red one. This one leads to Bowser.
The Final Battle
A cutscene starts where Bowser and Bowser Jr. are enjoying a dip in a floating, sprite-shaped, slime-filled hot tub. Peach refuses to get in with the two, and sees the Grand Shine Sprite deep in the slime. Then, Mario arrives and angers Bowser and Bowser Jr. One last battle begins.
There is a final cutscene. Mario and Peach end up landing on the lone island outside Delfino Plaza. They're okay, but FLUDD has sustained heavy damage. He asks if he was of assistance, and deactivates.
However, Peach tells Mario to look up, and they see the Grand Shine Sprite return power to the Shine Gate, and it brings the Shine Sprites back! Everyone on Isle Delfino is celebrating!
Meanwhile, Bowser and Bowser Jr. are floating out away from Isle Delfino. Bowser tries to tell his son that Peach isn't his mother, but he's already figured that out, and has one thing on his mind: fighting Mario again someday. Bowser likes his enthusiasm and is proud of him, but wishes to take a rest.
That evening, Mario and Peach are looking at the sunset when Toadsworth and the Toad Brigade appear with a repaired FLUDD, once again his old helpful and reliable self. Then FLUDD says the last line of the game:
"The vacation starts now!"
You'll get to enjoy the credits, which show pictures of Mario, Peach, Toadsworth, and the Toad Brigade enjoying their vacation on Isle Delfino. Now that the isle is clean and the Shine Gate is back to normal again, Mario can have fun with his pals. He gets a little stir-crazy and jumps from the plane at the airport, and ends up falling and becomes very dizzy. Peach catches a fish, giving her a souvenir of the vacation. The Toad Brigade and FLUDD check out Noki Bay and take a group picture. Toadsworth chooses to ride Pinna Park's roller coaster, and gets the ride of his life!
After the group goes on their adventurous vacation, they relax and head back to Mushroom Kingdom. However, Il Piantissimo seems to have found E. Gadd's Magic Paintbrush...
If you've collected all 120 Shine Sprites there is a slightly different ending. You'll get a picture of Mario, Peach, and the gang inside Hotel Delfino's Casino, with most of the residents of Isle Delfino in the background. In addition your saved game in the start game area will have a sunshine logo next to it as a mark of your achievement.
Congratulations! You've finished the game!