World 1-4 (New Super Mario Bros.)
From the Super Mario Wiki
This level is a grassy meadow, much like World 1-1. The first ? Block (which is on the ground, and not floating, unlike most ? Blocks) contains either a Super Mushroom or Fire Flower, depending on Mario's form. A ? Block containing a Mini Mushroom is later found in the level. The Mini-Mushroom can be used to travel into a small pipe nearby that leads to the first Star Coin. A wall made up of Brick Blocks and cement blocks are blocking a Pipe. The wall is shaped in such a way that it's possible to throw a Koopa Shell on it, which will break all of the Blocks and will eventually allow the player to enter it. Alternatively, the player could use a Mega Mushroom to break the block wall. The pipe leads to a cave that contains a Star Coin that is reached by using a P Switch that turns every Brick Block into a coin and vice-versa. The third Star Coin is found in a small gap just before the level's flagpole that is only accessible by running through as Mini Mario, Shell Mario, or Mario without a Super Mushroom, or by sliding down the slope and through the gap in any form.