South Park - Stan's garage
In Stan's garage you'll find a Guitar Hero controller, which is a reference to the episode 'Guitar Queer-O' (S11E13) in which Stan and Kyle become obsessed with the game.
South Park - Kenny's closet
Kenny's closet contains references to, among others, 'Cartman Joins NAMBLA' (S4E6), 'Jewbilee' (S3E9), and 'Mysterion Rises' (S14E12).
South Park - Clyde's garage
A memorial to Clyde's mother can be found in his garage, referencing 'Reverse Cowgirl' (S16E1) where she's sucked into a toilet and killed after he forgets to put the seat down.
South Park - Stan's closet
Try to open Stan's closet and you'll hear Tom Cruise say "I'm never coming out!", which is a reference to the controversial episode 'Trapped in the Closet' (S9E12).
South Park - the Sea People
You'll find a jar of Sea People on Cartman's desk in his bedroom, referencing the episode 'Simpsons Already Did It' (S6E7).
South Park - Cartman's closet
Cartman's closet references 'AWESOM-O' (S8E5), 'Weight Gain 4000' (S1E2), 'Trapper Keeper' (S4E12), and 'Christian Rock Hard' (S7E9).
South Park - the lair of Professor Chaos
In one of the public storage lockers you'll find the lair of Professor Chaos and his sidekick, General Disarray, from 'The Coon' (S13E2).
South Park - the Mysterion Mystery
On one of the walls you'll see references to The Coon and Professor Chaos trying to uncover the identity of Mysterion, which was revealed in 'Mysterion Rises' (S14E12).
South Park - Kyle's closet
Kyle's closet contains references to 'Chef Aid' (S2E14), 'An Elephant Makes Love to a Pig' (S1E5), and 'Stanley's Cup' (S10E14).
South Park - Mackey's storage locker
The piles of junk in Mr. Mackey's storage locker references 'Insheeption' (S14E10) in which Stan and Mackey develop a compulsive hoarding habit.
South Park - the cinema
The cinema is showing Asses of Fire 2, a sequel to the Terence and Philip film that was the focus of the South Park movie, 'Bigger, Longer and Uncut'.
South Park - the Photo Dojo
Photos on the wall in the Photo Dojo shop reference, among others, 'Towelie' (S5E8), 'Cartman Sucks' (S11E2), and 'Imaginationland' (S11E10).
South Park - the town notice board
The town notice board is packed with references including 'A Scause for Applause' (S16E13), 'Timmy 2000' (S4E3), and 'The Last of the Meheecans' (S15E9).
South Park - City Sushi
The destroyed City Sushi building from 'City Sushi' (S15E8) can be found near City Wok and the Tower of Peace.
South Park - Jimbo's Guns
The stuffed corpse of Scuzzlebutt from classic episode Volcano (S1E3) can be found inside Jimbo's Guns.