GMod LAN parties. To be fair, I'm sure I could have the same fun now. Only everyone's schedules are different and we can't get together for a good 24 hours of intermittent sleep, play, and loud music.
I feel like I spent a good portion of my time walking and finding ways across rough terrain even after all the fancy gear was unlocked. The motorcycle could get you maybe half the way, usually.
I mean, at least until the zip-lines. Those ruined the game. Honestly, the rebuildable roads were a bad inclusion as well. Sitting on top of a hill, looking down at the streams and terrain around you, figuring out the best route with your tools, was peak satisfaction in that game.
Majesty (Majesty 2 is okay, but lacks the charm of the original, but YMMV) - you run a kingdom full of heroes. The catch? You don't command the heroes. They have their own AI and goals and you have to offer incentives and place the necessary buildings appropriately to both enable and encourage them to do their jobs of saving the kingdom.
Ronin - a stealth/platformer. Combat is turn-based. No, combat is not mechanically separate from the stealth OR the platforming. Relatively short but very fascinating.
Pawnbarian - Roguelike, but movement and combat is done by chess rules.
Exanima. Combat is based entirely around physics/momentum and positioning. It's hard to get the hang of, but is immensely satisfying once you get your "He's starting to believe" Matrix moment and successfully block a few attacks in a row.
Crusader Kings 3. You know those map-painting Grand Strategy games, where the goal is to conquer other territories? One of those, but you're running a noble dynasty whose fortunes rise and fall, even passing between the overlordship of different countries and kingdoms. A lot of personality. I guess it's not as innovative as it once was, since it's spawned imitators at this point. Hm.
Ring of Pain. It's... hard to describe.
Phasmophobia. Multiplayer only. You hunt ghosts. Not like, 'combat' hunt ghosts, like 'You need to find evidence of ghosts' hunt ghosts. But the ghosts definitely hunt you back - in a much more malicious way.
Death Stranding. Walking simulator. No, not like 'You don't do anything but hold down the walk button', like 'You need to keep your balance while carrying things' walking simulator. Immensely weird.
Star Trek: Bridge Crew. Multiplayer only (at least practically speaking). Each person plays a separate member of the titular bridge crew, and cooperation to achieve even simple tasks is key.
Gods Will Be Watching. A series of puzzle scenarios about calculated risk, failure, and learning the rules anew each time.