Puyo Pop

To access a hidden sound test in the Options menu, defeat Kaakun twice in Trial Labyrinth to receive CARD 18.

Select 18 in Gallery mode in the Options menu, and you'll be presented with the Sonic Team artwork for the 5 Puyo. Every time you press 'A', you will be greeted with another sound effect, including the 'gloop' sound of puyo and ojyama being dropped, which is curiously absent from the game itself.

Signs let you gain access to new areas.

The WELCOME SIGN lets you into Hajimari Forest; you start with it.
The MERCHANT SIGN lets you into Goldtown; win from Seriri in Hajimari Forest.
The WARRIOR SIGN lets you into Gachinko Dungeon; win from Nohoho in Goldtown
The CONJURER SIGN lets you into Magical Tower; win from Draco Centaur in Gachinko Dungeon.
The MONARCH SIGN lets you into G. Dark Prince Castle; win from Witch in Magical Tower.

The last sign is the hardest to get.
Defeat Satan-sama twice to get the DUBIOUS BOOK.
Defeat Witch in magical Tower with the BOOK to get FLAME EXPANDER.
Defeat Draco Centaur in Gachinko Dungeon with FLAME to get NOHOHO RUCKSACK.
Defeat Nohoho in Goldtown with the RUCKSACK and get SCALE PENDANT.
Defeat Seriri in Hajimari Forest with the PENDANT and she'll give you the GODDESS SIGN, which lets you into Trial Labyrinth, the 6th and final area.