Save the game at any point to unlock a "Challenge Mode" option at the main menu. Select that option to replay previously completed sections in a time attack type scenario. The objective is to collect as many Pikmin as possible before nightfall.
Fireworks
Just press down on the d-pad and wait for four Pikmin to carry you to their onion. You will not get sucked in to the onion, but you will get close.
30 Frustrating Parts
It's hard to gather all 30 parts for Olimar's ship, so it's best to gather the parts that are most important. He specifically says he doesn't need all 30 parts. Read the description for it before you have your Pikmin take it back to base.
Getting Various Colored Pikmin
You can get the Red Pikmin when you start the game (Impact Site). You can get Yellow Pikmin when you get to the next area (Forest Of Hope). You can get the Blue Pikmin in the third area (Forest Navel).
A Bunch of Flower Pikmin
Have any Pikmin bring something back to the onion and when they return pikmin seeds come out. Go somewhere (anywhere) and then come back (before nightfall) and check the seeds. They will be buds or flowers! I got a bunch of flower pikmin!
Captain Olimar can Fight
If you are tired of losing pikmin to those little creatures,(smaller ladybug lookin things, those large tadpoles with the hugh mouths, and those little grubs that break down bridges and fly), just walk around with Captain Olimar and when you come to one, just press the A button repeatedly (while moving around so not to get hit), this results in a dead bug and no dead pikmin.
Defeating Frogs
Get a large group of Pikmin (preferably Red). Go up to the frog and dodge his first attack. Then, swarm him with your Pikmin. He will not be able to get off the ground, causing a near instant kill. This works with all frogs, but requires more with the Distant Spring frogs.
Easy way to kill a Bulborb
When you see a bulborb don't just attack it from the front. Get all of your pikmin, then sneak behind it and use the C stick. When they start to attack it will only be around 2 seconds before it dies.
Defeat Bulbax
Take 20 Yellow and 80 Red Pikmin on a new day to fight him. Get bomb rocks for all your Yellow Pikmin from the back and then put all you Pikmin outsdie the Arena. Wake him up. Now leave a Yellow bomb rock Pikmin for him to eat. Wait for the bomb to blow up in his stomach and swarm him. When he shakes them off, call them back and repeat.
Inivincibility
Press Down and the D-Pad. Olimar cannot control anything but cannot be hurt.
Get Olimar Turned into Pikmin
Fail to get all the required parts in 30 days.
Defeat Smoky Progg
Visit the Distant Spring before day 15 and find an egg in the huge lake. Smash it and run back to base. The Smoky Progg will now chase you. Any Pikmin (even planted) that touches the slime it leaves will die do use Red Pikmin and swarm his face. If you kill it he will leave a nut that will make 100 Pikmin.
Unlock the Final Trail
Unlock the FINAL TRAIL when you get all other 29 spaceship parts.
Aternate Ending
The ending you get depends on how many parts you get:
Bad: if you fail to collect the 25 REQUIRED parts of Olimar's ship, he will be stranded forever and become a pikmin.
Normal: If you get the REQUIRED parts of Olimar's ship, he will leave the planet for home.
Great: After you collect the secret safe Olimar will return the planet happy that his ship was repared %100 and there is a little surprise at the end.
