Hi! I'm the girlfriend. :D
We are playing Moen today with another friend and we found some more Secrets.
Here a nice feature, which is not exactly a spoiler:
With "L2 + left on the control pad" you can change your hair colour. Right now there are 9 different colours.
- - - - - S P O I L E R S A H E A D - - - - - -
At first we found the Secret Passage to the "Ball Room" and threw everything outside. (You can access the room near the soccer field when playing with two or more players. You have to crouch to go inside. With two players you need the air bug that I explain next in this comment.)
You can "fly" / bug yourself in the air, when opening the emote menu (with right on the control pad) without choosing an emote. Then jump and while in the air choose an emote (press right again).
On the left of your spawn point you can find a place to play basketball and a pink crytal.
On the right from your spawn you can find a (let's call it) "golden coconut". It makes some sound when being kicked. Further to the right through the water you can find a tunnel / short cut.
What Bacon mentioned above is a pretty huge cave that you can access through climbing from the left side of the island or at a point somewhere in the middle of the course where you have to make your first crouch jump from one stone to another. Through the short cut fom the right side of the island (through the water) you end up exactly at that point. Inside the cave there are two stone figures.
Some of these were things we already knew yesterday, but I wanted to sum them up somewhere so others can find them too.
Well. I hope that might be helpful to someone here, but I hope you could find these secrets by yourself or at least read these spoilers after accessing the highest point.
The game itself is awesome and I (/we) enjoyed it very much. Keep up the good work!