Become invisible - In battle, get all of your team members KO-ed except for Nei. Then get Nei to cast Nasak and she should KO herself. Then, your characters will turn invisible and you can walk around dungeons freely.
(This works in all version of PS2)

Make the game scroll one frame at a time.

When paused, press C and one frame should go by.

Slow motion play - When paused, press and hold B and play as normal. The game should play very slow.

Switch game music - This trick allows you to have different music play in other areas, using the slow motion code. First, you go to the area that has the music you want, like the weapons shop for instance. Next, press start and hold B.

The game will move in slow motion with no sound, so you can still move. Now, then go to another place and let go of B, then press Start. The music from the place where you first pressed B and Start now plays in this area!