EVE Online is an online space exploration MMO with elements of strategic combat and life sims — you could spend all your free time on just this game. Not all of it is free, but the opening free portions should keep most gamers busy for quite a while and let you discover if the full game is for you. Gameplay primarily consists of navigating space in your ship.
For free, you can pilot small vessels to explore space, mine for resources, trade, get into fights and otherwise do what most players can do who buy subscriptions. Free accounts won’t have access to large late-game ships, but filling in everything you can for free still represents a large amount of content.
EVE’s world is unique among MMOs in that most players exist in one very large game world across several star systems. EVE also stands out by keeping track of character progress passively in real time, giving you points into different categories you’ve queued up whether you’re logged in or not.