Elite Dangerous is my go-to lately.

It's different to most other games, by not being goal-oriented except for the goals you set for yourself. No main quest line dictating progress. No mandatory tasks. No win condition. Instead, it drops you into a simulation of our entire galaxy roughly 1300 years in the future, where humanity has mastered hyperspace travel and spread through hundreds of star systems.

(To give an idea of the simulation's scope: Around 85 million systems have been recorded by players so far, and those are a vanishingly small fraction of what's out there. Space is big.)

I like that it offers a variety of activities to fit whatever mood I might be in on a given day. I can hunt pirates, mine asteroids, engage in a bit of piracy myself, find and collect bio samples, infiltrate rival settlements, venture into vast unexplored areas of space, discover Earth-like worlds that nobody has ever encountered before, defend humanity against hostile forces, photograph beautiful stellar phenomena, rescue stranded survivors, customize and finely tune my ship to perform beyond its original specs, team up with friends, pledge to a political power and expand their influence, or chill out as a space trucker and haul cargo to earn enough money for my next upgrade. It can occupy all my attention, or just be relaxing entertainment while I listen to music or an audiobook.

It's an MMO in the sense of having a large game world (galaxy) shared by all players in real time, but PvP is optional. One mode exposes you to other players, while another limits you to NPC encounters. You can switch between them at will.

One warning: A space ship has more than a few controls to learn, and they're better suited to a game controller or HOTAS than a keyboard and mouse. I use button combinations for almost everything beyond basic flight controls, since there aren't enough buttons on a controller for everything.