Items are listed in the order that you can find them!

Book 1/4:
As soon as you enter the main hall after picking up the guest pass, at the bottom of the stairs to your immediate left.

Teacup 1/4:
Upstairs in the Carrion room.

Bag 1/4:
To the right of the Weird West room's entrance.

Teacup 2/4:
On the side of the Weird West room.

Book 2/4:
Directly in front of you after riding the small elevator up to the floor above.

Teacup 3/4:
On the table of the room where the credits are playing after bypassing the lasers.

Book 3/4:
A dead end in the vents before falling outside into the next hall.

Book 4/4:
On a table in the Blightbound room.

Bag 2/4:
Hanging off the side of the harpoon for the Olija room.

Bag 3/4:
Near where the Serious Sam trailer is playing.

Teacup 4/4:
Fall guys upstairs section, to the left of the trampoline.

Bag 4/4:
On the statue as you are leaving the Shadow Warrior room.

For each set of secret items collected it'll change one of the thumbnails on the website page that's shown once you leave the convention: