Burning Plants
In the Ice Palace, and in a few other places, there are plants. If you use the Fire Scroll, you can start them on fire. This causes repeating damage in increments of 8.
Giant Plants
On Twinpeak Mountain, there are some Blue Plants coming out of pots. Walk up to them and use the Water Scroll. Throw a bunch of bubbles on them and eventually they will grow to about 3x the normal size. It's fun to do, but doesn't seem to have any effect upon the game.
Mom Minku
Once you have captured all the minku, go into the entrance to Meandering Forest. The Mother Minku will be there. Be sure to stock up on BP and HP restoring food.
Unlimited Money
In the village bar, which is open in game time from 6pm until 2am, you can play a high-low game with a character named Macho.
The game costs 100 to play and will return doubled amounts in this sequence: 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200, and lastly 102400. However if you get 102400 he will no longer play the game with you.
If he picks a number less than 7 then select high and if he picks 7 or higher pick low. Starting with just a few hundred or so for a loss cushion you can quickly become extremely wealthy. Always stop your winnings at 51200 though or you can't keep raking it in!
Incidentally, time doesn't pass in the game while you are doing this so knock yourself out!