New Super Mario Bros. U: Acorn Plains-5 -- Rise of the Piranha Plants

This area has lots of Giant Piranhas. The Fire Flower is very helpful here. There are coins under the area near the rising Piranha. The first Star Coin floats over a giant Piranha Plant. Baby Yoshi can help you get it. There's a Hammer Bro in front of the midpoint. The path ahead is slanted and has more giant Piranhas. After them is another Hammer Bro. There's a pipe blocked by a Piranha Plant. Defeat the Piranha, then go through the pipe to an area with the second Star Coin over a couple of giant Piranha Plants. Return to the main level and avoid the Piranha Plants on the ceiling. The third Star Coin is guarded by a Hammer Bro. There is slanted land to the flagpole. This opens the path to the castle and a Mushroom House. Time your jump over the Piranha Plant in the pipe to avoid dealing with it, or backtrack and go past Acorn Plains-4.