While not a mod, the base game allows for "secret" item combinations in the combinator that feel like mods: Pickle Rick:
By default, shooting a portal at a wall that isn’t a designated "portal zone" does nothing. This mod (unstable, use with a backup save) forces portal physics onto any flat surface. Want to shoot a portal under the couch and crawl into the backrooms of the game engine? This mod lets you. Warning: You will fall out of the world map. You will see the un-textured skeletons of models. You will understand the cosmic horror of unfinished code. Rick And Morty Virtual Rick-ality Mods