The Legend of Zelda: Four Swords Adventures

Unlock Bonus Shadow Battle arenas in the Dark World:
Finish the Hyrulean Adventure

Beat the giant warriors
If you have the Roc's Feather jump over them and push B button to kill him.

Boomerang
Use this item to get out of reach items and gems.

Mini-games at Tingle's Tower:
Finish the 2nd stage of a level with 2 or more players.
or
Finish any world, then go back to it with two or more people.

World #1: Bucking Bronco mini-game
World #2: Cucco Wrangling mini-game
World #3: Hammer Tag mini-game
World #4: Monster Hunting mini-game
World #5: Volley by Golly mini-game
World #6: Mole Melee mini-game
World #7: Monster Hunting Returns mini-game
World #8: Bonus Level mini-game

Get Force Gems
After you get the Fire Rod, use it on several bushes. Every bushes will
reveal one Force Gem.

Big Purple Force Gem (worth 300)
Get the shovel in the 2nd area of stage 2. Dig behind the house with
the two pots on the top and many pots inside of it (southeast) to find
a big Purple Force Gem.

Hidden World
When you walking on level 2, you will find some special places, where you
can see flash and you can hear a certain sound when you are near them. After
obtained the Moon Perl, use on these special places to go into the
hidden world, where all of the children are located.

Weapon upgrades at Great Fairy Fountains:
Enter into A Great Fairy Fountain, to upgrade your current weapon.
You can upgrade to the followings:

Slingshot: Shoot 3 stones at the same time.
Shovel: Detect hidden passages and treasure with sound.
Roc's Feather: Double jump.
Pegasus Boots: Run over holes and pits.
Magic Hammer: Larger impact radius.
Lamp: No upgrade.
Fire Rod: Shoot flames in 4 directions.
Bow: Shoot 3 arrows at the same time.
Boomerang: Reach further distance.
Bomb: Makes the bomb bigger.

Tower Of Winds
Enter to the Dark World while playing the Tower Of Winds level in Shadow
Battle. You will find 4 switches. Hit all four swithces to drop the Giand
Bomb into the Light World. This bomb will destroy anything that remains.

Break the line of customers (area 2 stage 2)
Use the boots to run into the line.
The people will briefly move away.