Quest 64
Building Defense
While walking around a town, save your game at the town inn. Then, walk around until you find the hardest enemy you can find. Now all you have to do is let it hit you. Seriously! Don't attack it. When you die, you'll be back in the inn and you'll have all of the experience you gained before you died. Just save your gave and do it over and over and over...
Credit for Codes Goes To: Melaine B via N64cc

 Buliding Defense
 Hidden Room
 Hidden Spirit
 Full Power At Hotels
 Avoid Some Attacks
Hidden Room In Limelin Castle
First go to the throne room where the queen is waiting for you. Next find the wall that has what looks like a sun on it. Go up to that wall and it will slide open revealing a hidden room.

Hidden Spirit on Ferry
After you have beaten Nepty and have been transported back to Larapool, make your way back to the ferry that originally brought you there, get on the ferry and ride it three times. On the third trip, stay in the cabin and move up to the crates on the right. A "?" will appear over brian's head, signaling an invisible spirit element!

Full Powers At Hotels
To get full power without using items, go to a hotel and say you will stay the night. Then, when at the saving screen, hit B to escape. You will now have full health.

Avoid Some Attacks
When an enemy casts a ROCK spell on you, start running directly towards them. You will take control of your character while the boulder is still in the air. If you can get close enough to the enemy, fast enough, the stone will pass harmlessly overhead.
Credit for Codes Goes To: IGN 64