I haven't played it myself but I see Startopia get recommended a lot when people are looking for a fun space sim.
A once thriving network of space stations and planets have been left lifeless, redundant scars of the apocalypse. Into this age of darkness comes light. You will lead the way in rebuilding the network of space stations to a standard accepted by the individual alien races, attracting them to harness their expertise and research new technologies. Thus begins the hilarious battle of wits and cunning, full of daring escapades.
Key Features:
Fully functioning alien society, with racial animosity and friendships, crime and punishment, leisure and entertainment, and functioning economy
Unique 'Torus' play area gives a viewpoint never before seen in a game
16 habitable subsections spanning three decks in each space station
Bio-deck with editable landscape and manageable ecosystem allowing complete control of terra-forming
Multiplayer support for up to three other stationmasters
Varied levels of play, from simple sandbox to detailed station micromanagement