You can run Doom inside (DOS) Doom, for real.

Technically, this hack leverages an exploit in the DOS version of Doom II, a bigger and better sequel that arrived a year after the original, and manages to get Chocolate Doom (a modern port of the game that’s compatible with the original DOS version of the game, occasionally referred to as Vanilla Doom) working within Doom as an animated texture. The modder’s 15-minute video goes into extreme detail about how this hack works, and how kgsws created a custom movie theater Doom level so others can try it out, too. You can grab their custom code from GitHub, and find a copy of the DOS version of Doom II on Steam.