After passing through the secret tunnel from Threed to Grapefruit Falls, you discover a strange village.
Just outside of the secret passage is a man selling some simple healing items. However, there is a Magic Butterfly spawn point just north of that man, so go in and out of the cave to respawn the butterfly so you can heal up for free.
Make your Way North to Saturn Valley
Go northeast along the river. Be sure to get the Bomb on the left. Farther north, you will find a Protein Drink. Just north of that, you find a cave entrance on the right. Go inside.
Go to the right, and enter the small room on the way to get a Bottle Rocket. Then keep going to the right through the cave to reach Saturn Valley.
Visit the Saturn Valley Hospital if you have a mini-ghost. Dr. Saturn will get rid of it for you.
You can get a Rust Promoter from the gift box on top of the ladder in the northeast of town.
The north wall of Saturn Valley has a couple of cave entrances. Go into the one on the right to find three trash cans. Each one contains an item.
Then go into the cave on the left. Talk to one of the Mr. Saturns to learn how to enter the Grapefruit Falls secret base. The secret is, when the guard asks you for the password, stand still and wait for three minutes. You have to talk to this particular Mr. Saturn in order for it to work, though!
Go up the ladder in that cave, then go into the building outside. You can buy new equipment there. However, don't get the Bionic Slingshot, because it misses 18.75% of the time. Get the Red Ribbon for Paula, then get Great Charms and Silver Bracelets as needed. The other Mr. Saturn sells healing items. The Peanut Cheese Bar recovers about 100 HP. The Refreshing Herb cures some status ailments, the Secret Herb cures more status ailments than the Refreshing Herb, and the Horn of Life revives an unconscious ally or can be used to heal most status ailments.
When you are done exploring the town, make sure that you have the Jar of Fly Honey that you got in Threed. Then leave town the way you came in.