The Gift Shop
From the pool, head right to speak to the Shopkeeper. Attempt to leave after the conversation, and he'll start another and give you a novelty size key.
Use the novelty size key to open the door on the left side of the pool.
Descend until you reach the broken well. Head left.
Pull and hold the lever here with one party member, and switch control to another.
With that other party member, drop into the space next to the broken well to find a lever below a platform. Pull this, and switch control to your third party member.
With both levers pulled, the gate will be raised. Head through, hop the spikes, and pull the third lever to lock the gate in place.
Head left to find a series of platforms. Start climbing.
You'll find your first Cave Painting on the way up.
Just across from the Cave Painting is another well. Use the crowbar (you hung on to it, right?) to break off the crank. If for some reason you dropped the crowbar before starting the Gift Shop level, it will be on the shore near the starting pool.
Grab the crank and return to the broken well you passed earlier. Attach the crank and turn it to destroy the well and free a bucket.
This also grants you access to another Cave Painting. Check it out, and grab the bucket. Return to the well where you got the crank.
Hop across the gap to find a sparking fuse cell. Place the bucket on the fuse box to catch the dripping water above, allowing you to safely touch the cell.
With cell in hand, head to the platforms under the well you destroyed a moment ago. There's a fuse box down here. Place the cell to activate the neighboring vending machines, and hit the hot dog machine to get a hot dog.
Now head to the platforms that lead up to the first fuse box. You'll spot a bell. Walk past it to find a pit of spikes. You can throw the hot dog into the pit, so do so.
Now return the cell used for the vending machines back to its original spot to power up a nearby claw.
With one party member, ring the bell next to the pit of spikes. This will summon the Crystal Monster, who will start munching on the planted hot dog.
With a second party member, activate the claw next to the fuse box to pull the Crystal Monster safely out of reach.
Proceed past where the Crystal Monster was to find another Cave Painting.
Continue to find a cliff. Dive off into the water below.
Head right to find a series of ladders. Climb down to find box of dynamite. Grab a stick and climb back up.
Light the stick with the nearby flame and drop it by the rubble to the right. After it clears, move forward.
Hop onto the upper ledge you come across to find a special item. What it is depends on your previous playthroughs (if you've had any), but pick it up anyway.
Just past this first item, have a second party member drop down to find a small tunnel of water. Swim through it to reach a second key item.
Keep moving to find two pressure pads and a door. Stand one party member on the first pad to open the door, and send a second member to stand on the other pad. This will hold the door open for the other two.
Just past this, you'll spot a ladder. Climb it to find a third key item; pick it up with your last party member with a free hand.
Move a little farther to find an elevator call button. Hit it and ride the elevator up to return to the vending machines from earlier.
Return to the Shopkeeper with all three trinkets to enter the cave proper.
Follow the path and jump onto the first rope to raise a weight across the room.
Keeping the first character on the rope, use a second party member to jump to the other side of the room. You'll find a Cave Painting over here.
Drop to the ground and head right to find a crate; you'll have to let go of the first rope to reach it. Drag it under the gap between the weights to create a safe path down for the rest of your party.
Have all three members stand on the pressure pad to the right to open the door. This grants you access to the next area.