Beat the game, and before the credits start, hold down the R and L buttons and release after the credits appear. You will hear "Tomorrow is a Holiday," the other ending instead of the normal credits theme.
Emulator: View title in Japanese
Note: This trick requires the ZSnes Emulator and the game's ROM. Press 5 at the title screen. This should remove the English title and show the Japanese title. Press 6 to return to the original title.
Hard mode
Highlight "Continue" or "New Game" at the title screen, then press A + B + X + Y. A voice will say "Yatta!" to confirm correct code entry.
Sage/Savory Stat Boost Calculation Error
The items Sage and Savory boost your maximum HP and TP by 5% respectively (10% with the Red varieties you obtain by using a Rune Bottle on them first). However, this boost doesn't account for the fact that Accessories can boost these stats. For example, if you have 1,000 HP, a Black Onyx equipped boosts this to 1,300. If you use a Red Sage without it equipped, you gain 100 HP - but with it equipped you gain 130. Thus, to maximize the boosts from the Sage and Savory items, take advantage of this calculation oversight by equipping your best HP/TP-boosting Accessories before you use the item.