Easter Eggs in the game and their free rewards
Misty Swamp
Underground Garden
Enter the room to the left of the entrance. Near the bottom center of this room, slightly to the right (you can see a slightly round edge), you will find a hidden passage to jump to. The wings are at the end of this passage.
Pyramid Level 2
Cauflax Tomb
At Level 2, at the bottom left of the map. Jump over the sarcophagus in the top left to enter the hidden passage. The wings are at the end of this passage.
Steam Train
Arm's Storage
From the entrance, to straight down to the first catapult and press your interaction key (usually F). You will get flung to another zone. In the center of the room awaits your reward.
Dawn's Chapel
Left from the stash. Jump up to the end of the half arch (11 o'clock from the stash). Follow the passage to the end to find the hat.
Outskirts of Inoya
Rat Den
From the entrance, you can either teleport straight up or go to the single room located horizontally in the center of the zone, one room down from the top, with a gap on its left side. In this room, there are four small alcoves. Jump up from the top right alcove to teleport to the item room and collect the hat at the end of the passage.