Turkey Sandwich:

To get to the sandwich, first jump down into this well here.
Go to the very end of the cave and you'll find a blue fridge.
Open the fridge and you'll find the sandwich is incased in ice, use Pyrokinesis to melt it and the sandwich is yours.

Gold Doubloon:

An easy one to find, it's hidden in a niche on the right side of the Main Hall.

Voodoo doll:

This one is inside the Main Hall building. Climb onto a table and use Levitate jump up until you climb onto one of the beams, the doll will be sitting on the beam above the stage.

Eagle's Claw:

First climb the tower shown here.
Then float down to the platform on the right with Levitation.
Alternatively you can float to the left platform and use the trapeze to get to the right.

Cherry Wood Pipe:

To the left of the GPC exit, follow the path around the hill.
You should see a plateau covered in trees and a GPC sign, Levitate over there.
The pipe is infront of the truck in the overgrown tree.

Condor Egg:

To get to the Egg first you have to go through this cave here. Be sure to have earned your Marksmanship Badge to deal with the bear that lives in the cave.
Once you reach the other end you'll come across some rails, grind on the left rail to reach the egg.

Dinosaur Bone:

That big dead tree on the top of the hill is where you'll find the dinosaur bone
Use Levitate to climb the branches up to the bone.

Fertility Idol:

Just left from the janitors trailer is a tree a beehive hanging from it. A bear might also be nearby so be careful.
Use your Psyblast to destroy the beehive for the Idol.

Fossil:

Once inside of Sasha's secret lab, there is a large ledge where the fossil is placed. Use Levitation to reach it.

Golden Acorn:

To the left of the exit to the Boathouse is the Golden Acorn and a squirrel who guards it.
You CAN NOT kill the squirrel or use telekinesis to grab it. You need invisibility to sneak up and steal the acorn.

Miners Skull:

If you continue to go left from where the Golden Acorn is, you'll find 2 geysers with the Miners Skull stuck in one of them. You'll need the Shield ability to block the first geyser and dislodge the skull.

Pirates Telescope:

Also to the left of the Boathouse exit is the telescope. To reach it you'll need Levitation to climb the huge rocks and reach the vines up top.

Glass Eye:

Follow the river to the GCP and you'll end up at a storm drain with the eye behind the bars. Use Telekinesis to grab it.

Diver's Helmet:

Bounce your way up onto Boathouse's roof using levitation.
Go along the right ledge and you'll find a cave with the Divers Helmet inside.
Caution! A cougar will move into the cave once nightfalls so try to grab it before then.

Psychonauts Comic #1:

Once you reach the beach turn right into this niche here and climb onto the ledge.
Continue going along the wall until you see a couple of branches, the comic is underneath.

Gold Watch:

Once you have resolved the issue with the guard you'll see a statue in the center of the asylum courtyard, sitting on the very top of it is the gold watch. Use Levitation to reach it.