World Bowser contains a single Stamp to collect in every level.
After the Checkpoint Flag, run to the left in the direction of the spikes. At the very end is the Stamp.
Have Plessie ride right between the two Bowser sculptures to ride along a line of hidden clouds. The Stamp will be at the end.
When you first see the first row of spiked ants, drop into the hole they are climbing from to find the Stamp below. This will take you to spinning wheel with a Green Star at the end.
The Stamp can be seen on the roof of one of the carts. Ground Pound the Question Block on the right to have it expand right to the roof.
On your way to the end, there will be a platform with a glowing center. Jump to the right to find a hidden path that takes you to the Stamp.
When you first make contact with the floating blobs of water, swim to the top and wait for it to hit the roof. The Stamp will be above one of the large spiked objects.
Luigi required! On the first moving platform, you’ll come across a Luigi switch. Hop on it as Luigi to earn the Stamp for this level!
After the Checkpoint Flag, ride down the twisting platform until you see a Hammer Bro. on a small platform. Jump on him to grab the Stamp and safely land.
After you defeat Meowser, climb the bell tree to the left of the flagpole to find the Stamp on top.
Enter the tent in World Bowser for a free of charge Stamp!