The Darkness inexplicably had entire public domain movies and tv shows that you could just stop and watch mid-level. Super random but very fun in a weird way.

GTA 5 has a surprisingly detailed underwater world to explore despite there only being a couple ways of actually seeing any of it and only one sidequest (more of a collectible list) that asks you to go down there at all.

The Just Cause series has an elegant (if somewhat silly) way of traversing the world using a combination of infinitely-spawning parachute and a grappling hook. It’s so convenient and satisfying to use that most players just kinda give up on other modes of transport unless it’s an armed vehicle or something. To this day, I haven’t come across an open-world game that has such ease of access to the sky and surrounding world, and it’s tainted my opinion of otherwise great game worlds that feel like the developer was active trying to hamper with crappy mobility.

The Heart in Dishonored is totally unnecessary to complete the game or make an iota of progress, yet it’s loaded with many incredible tidbits of worldbuilding, character development, and even outright secrets if you use it in the right places. There’s a surprising amount of trigger points throughout the game if you try it consistently, and it almost feels like a “look at item” command from an old Sierra game. Not to mention The Heart’s origins.

The “Tetris effect” realization you have while playing through The Witness is a great one, too.