This is the list of items in the game, New Super Mario Bros.: Clash of the Koopas. Note that Mario actually means all the playable characters in the game. For example, Mario's not the only one that can use the items, but the other playable characters (Luigi, for example) can use them also.
Turns Super Mario into Mega Mario for a limited time to let him turn gigantic, destroying enemies, blocks, and shrink and destroy pipes. In this form, he can't spin jump or wall jump. When he ground pounds, Goombas fall out of the sky. If he's in a form other than Super Mario and Small Mario and he collects a Mega Mushroom, when he goes back to normal, he returns to his super form.
Acts the same as the Blue POW Blocks, but it can't be held and floating in the air and it doesn't defeat enemies. It also gets coins out of Question Blocks and Brick Blocks and forms a chain reaction when another one is activated.
Appears after the player dies 8 times in a row in a level. When hit, the player(s) decide if the computer player(s) can complete the level for the player(s). The computer player(s) are the character(s) the player(s) are playing as. They don't run, but they walk. After the computer player(s) complete the level, the player(s) decide if he/she/they want to complete the level by themselves. If not, the player(s) will move on to the next level, and the scores won't be saved and the level will remain incomplete.
8 of these appear for a limited time when a Red Ring is touched. When Mario collects all of them, he gets a power-up (if he's in his small form or his super form), or a 1-Up Mushroom (if he has a power-up).
Turns Super Mario into Raccoon Mario to let him glide/fly (height and speed depends on the P-Meter), and swing his tail.
* - When Luigi, Blue Toad, Baby Luigi, or a Mii with green/blue as its favorite color collects a Gold Flower, they will turn silver instead of gold. When Mario, Yellow Toad, Geno, or a Mii that doesn't have green/blue as its favorite color collects a Gold Flower, they will turn gold instead of silver.
** - The Roulette Block contains every mushroom except for the Poison Mushroom.