Offline and couch co-op are not the same thing
Almost every "couch co-op" list assumes you have working internet and simply prefer to sit together. That covers the wrong situation. The one people actually search for is the connection being unavailable - a holiday cottage, a power cut, a hotel, a house move, a train, a rural spot where the wifi is a rumor.
In that situation a surprising number of "offline" games turn out not to be. Some want to verify a license before they start. Some queue a mandatory update the first time you launch them. Some are perfectly playable but bury local multiplayer behind a menu that expects an account. Which is why the honest version of this list is shorter than the couch co-op one, and why it leans heavily on a single platform.
The Switch is the answer, and it is worth saying plainly
Nintendo builds four-player local multiplayer into almost everything it publishes, ships games on cartridges that do not need to phone home, and makes a machine you can carry to wherever the internet is not. Five of the ten picks below are Switch games and that is not a coincidence - it is the platform designed for this exact scenario.
Mario Kart 8 Deluxe is the single best offline group game there is: four players in split-screen, no learning curve, no account, no connection, and a cup takes fifteen minutes. Super Smash Bros. Ultimate goes to eight players locally. Super Mario Party Jamboree is a full evening for four. Super Mario Bros. Wonder is the four-player platformer that works for mixed ages, and Nintendo Switch Sports gets people off the sofa.
What to check before you rely on a game being offline
Buy physical if the trip matters. A cartridge or a disc is the only version that definitely works without the machine having been online first. A digital purchase usually works offline too, but it has to be downloaded in advance and the console generally wants to have seen the internet at some point to confirm the license.
Update it at home. Consoles and PCs queue patches the moment they connect, and the first launch of a fresh install is exactly when that happens. Install the game, launch it once, and let it update before you leave.
Sign in first, then go offline. Digital licenses on both major consoles behave much better if the account has signed in on that machine before the connection disappears.
Bring the controllers. The most common failure of an offline game night is not software. Four-player local play needs four controllers, and they need charging.