World 1 contains six Stamps to collect in total.
Before you grab the Mega Mushroom and destroy everything, enter the green pipe to the left of the tall bell tree. Inside, enter the clear pipe and exit through the top to snag the Stamp.
As you pass through one on the clear pipes, you’ll notice the Stamp in a hidden area above. Return back through the pipe to the area with the single long block. Hop on top and jump to reveal another hidden block. Do this again until you can reach the top of the ledge. Enter the clear pipe on the wall to have it take you to the room with the Stamp.
Right after you snag the second Green Star, walk to the right and you’ll see the Stamp in a small gap. Wall jump up the gap to grab it, or walk over to the edge and drop down to get it.
In the section with the curved waterway and the small fences, you’ll see the Stamp right on the bend of the river. Make a sharp turn to snag it without falling off!
Right before the Flagpole is a series of tiles. Have them all turn yellow to lower the platform with the Stamp on it.
To the right of the arrow sign are four giant brick blocks. Use the nearby soccer bombs to break through the blocks to reveal the Stamp.