In Super Mario Maker 2 the story mode is pretty straight forward, but there are some secret areas that you can find if you do a little exploring and speaking with different characters. Some are pretty straight forward with characters that reveal themselves on the map or pipes that lead you underground. However, there is a hidden block that you can access to reach the Soundfrog who is hanging out in the clouds above the castle.
If you head to the far right of the castle area you and jump around you open a block that contains a vine that leads into the sky. Up here is where Soundfrog is and they’ll give you a handful of unique jobs to perform with the final one unlocking the Frog Cap hat.
To get this block to appear you will need to have spoken and completed the jobs for Partrick in the underground pipe on the right side of the Story Mode map. Partrick will place a block with coins on the castle, but this secret block that leads to Soundfrog is his other gift.
The Soundfrog won’t just give you the hat for completing his jobs. You’ll also get some unique scenery for the castle area. He’ll unlock both mountains and flowers to make the castle area even more colorful and scenic.