Cappy in a Haystack

Cappy in a Haystack is a side quest in the Fallout 4 add-on Nuka-World.

Detailed walkthrough
Between the Parlor Dinner Theater and Fizztop Grille, Sierra Petrovita is encountered talking to a raider. Intercepting the conversation reveals that she has come to find the reward for finding hidden Cappys throughout the park. Given that the reward was meeting the creator of Nuka-Cola, she figures that in his office might be the original recipe for the drink. She gives her sunglasses to you before leaving. Once you get close to a quest marker, it disappears, encouraging a thorough search of an area.

The "hidden Cappy" figures are paintings on concrete surface in public areas of the park. Without the Cappy glasses, the player sees only a fuzzy Cappy figure with a blurred speech bubble; with the glasses, the code is revealed:

Click on the revealed figure to register the code and update the quest. Note that the Cappy glasses do not work when worn under power armor. To see the code, exit the power armor, click on the hidden Cappy, then re-enter power armor.

Fizztop Grille
One is west of the Fizztop Grille, on the large wall beside the restrooms building.

Safari Adventure
The first Cappy in Safari Adventure is through the hedge maze beneath the treehouse, against a wall on the eastern edge. From the entrance, the order of turns is: right, right, left, left, and right. Watch out for the gatorclaws. The second is just outside the Primate house, on the back of the gorilla statue to the right of the door.

Dry Rock Gulch
The Dry Rock Gulch section has two; The first is located on the back of a gravestone next to the Giddyup Kid protectron. Another is located within the Mad Mulligan's Minecart Coaster, on the wall of a little shack beside the waterfall.

Galactic Zone
In Galactic Zone, there are two. The first is at the entrance to the dead traders' camp around the back of Star Control entrance. The sign should be on the left side entrance, right next to the Employees Only sign at the gate. The second is up the ramp right next to the exit door of the RobCo Battlezone. Turn right immediately at the top of the Spacewalk ramp, and the Cappy is on the right wall near the trash bins.

Kiddie Kingdom
There are two in the Kiddie Kingdom. One is in the Fun House in the turntable room, second door on the left when entering from the topsyturvy room. The second one is located right next to the moat to King Cola's Castle, up a stone staircase and into a destroyed wooden clocktower.

World of Refreshment
In the World of Refreshment, One is on the two story building across from the Nuka Wild sign in the wild west setting.

Return to Sierra Petrovita and use the keypad. In the office, there is a button behind the Nuka-Cola machine. This unlocks a vault to the head of the inventor. If one chooses to shut off the power and open the door, the head dies. Behind the door is a Nuka-nuke launcher as well as pre-War money, various Nuka-Colas, and some other loot.

If you pick the Nuka-World jumpsuit, you can still shut off power to Bradberton's head, and the door will open, though this causes Sierra to become hostile.

Rewards

 * Nuka-nuke launcher - Awarded after siding with John-Caleb Bradberton over Sierra Petrovita and cutting power to the Vault.
 * Nuka-World jumpsuit - Awarded upon completion of the quest after siding with Sierra Petrovita to keep John-Caleb Bradberton alive.

Bugs

 * Once after taking the elevator to the vault and following Sierra up the stairs Sierra will become stuck in the corner of the the stairwell. To fix this go completely out of the office and into Nuka-Town then re-enter building and return to the vault. Sierra will be talking to the founders head and you can continue the quest.
 * PC players can also use the console command to teleport her out of the corner and she will continue her path towards the door.
 * Once the inner door to the vault is opened, Sierra may stand in place instead of entering the vault. This can be fixed by closing the door again, which should then trigger Sierra to continue normally.
 * Sometimes unable to collect hidden Cappy in Safari adventure. The only fix is to reload to an earlier save file and attempt to collect again.