Free 1-ups:

Go to the dock near the entrance to "Ricco Harbor" and turn right. There will be a gap, a small platform, and then another gap. Jump in the second one and swim to the right. You should find a 1-up mushroom in a small crevice. Get it. Then go into any level and come back out. It will still be there. Do this as many times as you fancy.

More easy 1-ups:

Near the man who gives you the sunglasses are two umbrella topped tables. Go to the right of the one on the right and walk around a bit. You will step on a manhole. Do a smash on the manhole and it will lead you to a 1-up. Another can be found in front of the lady selling bananas. Smash the manhole, go into it and keep following the sewer until it gives you the option to turn towards the beach. You will come across nine coins. Search around a little and you will find an opening that will take you to a 1-up

Fast traveling:

Even if you don't have a turbo noose equipped, there is another way to gain speed. First start running. Then press B to dive. Keep pressing B to "re-dive." If the ground is wet (you can spray it yourself), hold B and you'll side even faster. This is good for the underground tunnels and most straight-aways.

Return to the Airport:

Once you've beaten the game (and saved), you will be able to go back to the Airport from the very beginning. It'll cost you, though. For the price of 10 coins, a boat close to the clock tower where you find Yoshi will take you to the Airport level -- which is filled with coins and fun stuff to do.

Hawaiian Shirt:

Want to see Mario in a different outfit than his trademark blue overalls and red shirt? Remember the guy wearing sunglasses? He'll also give you a Hawaiian shirt -- but you've got to do some work first: finish the game. If you've saved your game after you beat the last big boss, you'll be able to get your shirt anytime you talk to this guy.

Different ending screens:

Beat with game with 119 stars or less to get a screen where el Piantissimo checks out the paintbrush. If you finish it with all 120, though, you'll get an ending screen featuring every character in the game.

Trade 10 Blue Coins for a Shine:

In Delfino Plaza, go to the shop near shore leading out to the ocean (in direction of the Amusement Park.) After destroying the slimy piranha plant, the shop will arise to its normal state. Go inside to trade 10 blue coins for a shine any time.

Mysterious Red X's:

The red x's will lead you to a blue coin. Spray a red X, and it may show you a direction to a red or black X. Be quick, though - when a blue coin pops out from another end, it will fade in a short period of time.

Cannon Glitch:

Go to the entrance to Pinna Park BEFORE you have unlocked it. An easy way is to simply go into a new file. Then, ground pound on the worker's head. After a couple of tries, you should go under him and inside the cannon. Although it will not fire, you can jump to escape.