Demon Deconstruction
As you play through the "Lazarus Labs" mission, make your way to the room with the DOOM Guys crypt. Along the left side, you'll find a cabinet you can interact with to obtain the "Time Well Spent" challenge on the mission. The gameplay resembles Bejeweled.
Skyrim Nod
Explore the Kadingir Sanctum, near where you first meet the Baron of Hell. There, you'll find a cave with a hidden lever and a skeleton. The skeleton is wearing the Dovahkiin horned helmet, and has an arrow in its knee, a reference to the popular meme.
Super Turbo Turkey Puncher Three
Play through the "A Brighter Tomorrow" mission until you near the location where you find the challenge mission artifact. Look behind some fallen debris in the vicinity and you should find an arcade machine. It makes vaguely retro sounds when you are nearby, so listen for that noise if you're having trouble finding it. The cabinet lets you play Super Turbo Turkey Puncher 3, a mini-game wherein you punch turkeys to earn points.
Terminator Two Reference
In the Foundry level, it's possible to fall into molten liquid and die. If that does happen, your character will give a thumbs up sign as he sinks into the soup, a reference to events depicted in the classic sci-fi film, Terminator Two.
Demon Deconstruction...