Throughout the first area of Wet-Dry World, there are five hidden secrets. They come in the form of things you have to do and upon triggering one of the secrets, a red number will pop up. Keeping the water level at its lowest is ideal for this, since a few of them are on ground level. At the beginning of the course, stay on the ground level and head to the far end, where you should find a box that you can push up against the wall. You should also see a shadow of a yellow box on the ground, so you want to push the box towards the yellow block, trigger Secret #1. Breaking open the yellow box will be Secret #2.
Jump onto the main structure and you should see the designations of another pushable block near the Heave-Ho, so go ahead and push it to trigger Secret #3. For the next two secrets, you might want to raise the water level until you can reach the third tier of the main structure in the first area. Here, you should find a small raised platform that has an Amp circling it. Jump on this platform and break open the yellow box for Secret #4. Run over to the smaller cage and hit the purple switch, then jump up to the top of the cage, where you can break open the yellow box there for Secret #5. The star will spawn on the long wooden platform being supported by wooden pillars.