Secret Scene:
Talk to Paulas father in Twoson, then visit the hotel. When you leave the hotel a bonus scene involving Paulas father will be shown.

Avoid battles:
If you go into a room with a large amount or any amount of enemies in it, you can exit the room and re-enter it and the amount of enemies changes sometimes even taking them all out of that room. This can eliminate time consuming fights.

$50 Easy in Twoson:
Outside of the hotel room in Twoson, there will be a man sitting at a table. Constantly speak to him (his response will change each time) until he decides to hand you $50 so you can leave him alone. It takes about 20 sessions of talking for him to fork it over, but it's well worth it, plus it'll make up for the night you spent at the hotel.

Infinite use of Rock Candy:
You can use the Rock Candy in battles an unlimited amount of times to increase your stats. You have to have it in one of the two very bottom spaces of one of your characters' inventories, and you have to use it with any condiment (preferably something like Sugar Packets or Jars of Delisauce, because they will increase one of your stats by 2 instead of one). This will allow you to get greater boosts in stats when you level up.

Exit Mouse in the Cave of the Past:
Note: This only works in the Super Famicom version of Earthbound(Mother 2). In Mother 2, you could use the Exit Mouse in the Cave of the Past, which was never intended obviously, since the feature is missing in Earthbound.