When you are on the title screen if tilt left and right fast enough then the wording will change and it will say something different.
On the title screen, if you wait long enough, little Wario noses will scuttle onto the screen. Rotating the GBA makes them scurry off the screen.
Once you complete Warioman you will unlock the Minigame Towers. I dont know if there is a boss tower.
Upon completing the game to a certain extent, you can unlock in-game items via "Gatcha Koron".
These unlockable stages will replace the ? marks on the Story menu.