Revolted sticks a '90s style shooter (with all the gangly character models, low quality sound effects, and 2D splatter sprites that that entails) inside Fallout 4. It's an impressive feat. It fundamentally changes how Fallout plays, stripping out the RPG systems, adding in jump puzzles, a chip tune soundtrack, and generally making Fallout just recognisable under all the changes.
To get the mod you should head over to Nexus Mods. Once installed you'll find a computer terminal on the second floor of the Concord speakeasy.
In Revolted you play the overseer of a vault where the door has been breached and monsters from the outside are pouring into your underground bunker. You have to fight through the creatures to the entrance to close the door and save the day.
It's not exactly The Mouse-Trap but I do love these games within games.