When you get to the open world map, find a place with weak enemies, wrap a rubberband on your left analog stick pressing up, another on circle (if that's the OK command) go to sleep, wake up with leveled up. [spoiler]only if you are over leveled for the enemies there and know you'll never die to them[/spoiler] I didn't do this but my friend did back in the day. After hanging outside for hours we went into his room and the game was basically playing by itself. I was like wtf. He said " hell yea I gained 5 levels" or something like that. Forgot how much exactly.

Just enjoy the story, dont really need any tips. Its a great game. I'm playing through it again for the 100th time haha

Grind Grind Grind, don't leave an area until you can one shot everything