Complete guide for Koishi's Mind Wanderer, as of the Full Release version.
Koishi's Memories
Rock
Heavy Thoughts...?
Appearance: Koishi becomes a large rock.
Passive Effect: None.
Action: (Q) A thudding sound plays.
Practical Uses: None.
Enter the red door at the top-right of the Nexus. Go east in the park area and look for two rows of slim trees that form a mini path. Walk between these trees until you are teleported to a lake area with a dock. Walk off the dock then make your way to the bottom-left corner of the lake. Interact with the sparkle at the bottom of the giant stone.
Balloons
Fun...?
Appearance: Koishi becomes a set of four balloons, each a different color.
Passive Effect: None.
Action: (Q) A beeping sound plays.
Practical Uses: None.
Enter the red door at the top-right of the Nexus. Go north-east in the park area to a playground. Interact with the shining object on the bench.
Small
Insignificant...?
Appearance: Koishi becomes smaller.
Passive Effect: Koishi can now fit through small gaps.
Action: (Q) A sound effect plays.
Practical Uses: Allows Koishi to reach certain areas. Required for an effect.
Enter the gray door at the bottom-right of the Nexus. There's a random chance of a sparkle spawning on one of the bridges to your left. Interact with this sparkle if it appears; exit and re-enter the area if the sparkle has not spawned.
Telephone
Phone Calls...?
Appearance: Koishi becomes a purple old-fashioned telephone.
Passive Effect: None.
Action: (Q) The phone rings and Koishi is teleported to the Nexus.
Practical Uses: Allows you to quickly return the Nexus.
Enter the gray door at the top-left of the Nexus. Go left two buildings then go south. You should come across a building with a red brick entrance and a glass door. Enter the building through this door; once you're inside, make your way to the sparkling object on the table and interact with it.
Cat
A good friend...?
Appearance: Koishi becomes a green-gray cat laying on a blue ball with a closed eye.
Passive Effect: None.
Action: (Q) Koishi meows.
Practical Uses: None.
(The Small Memory is needed.)
Enter the gray door at the top-left of the Nexus. Go right to the fork in the path then go down. You should see a building with a small hole at the bottom of the wall. Enter this gap using the Small memory, walk up the winding path on the other side, then interact with the sparkle in front of the mannequin.
Ghost
Dead...?
Appearance: Koishi becomes a ghost.
Passive Effect: None.
Action: (Q) A whooshing noise plays.
Practical Uses: None.
Enter the closet at the bottom-left of the Nexus. There is a dead-end with a grave in the middle roughly north-east from where you enter. Navigate your way to this grave then interact with the sparkle in front of the grave.
Ending
Collect all six Memories. Wake up and exit the bedroom.