Space Engineers is an enjoyable "Minecraft with spaceships" kinda' game with quite a few mods that add various blocks (some are useful, others are aesthetic), skyboxes, and scripts (add stuff to gameplay). You can play it as either a survival game or as a building sandbox. There are three categories of things that can be built (determines what blocks can be used and how big they'll big), small ships, large ships, and stations. My one gripe with the game is that the enemy AI on ships isn't particularly impressive (I haven't played since the planet update, so no idea how the alien ai is). If you are so inclined, you can also do some crazy cool stuff with sensors and timers, such as create rudimentary AI drones and automate various stuff.
Some screenshots since I like screenshots:
Edit: Just a heads up, your computer will need to be capable of running direct x 11 well to play on planets.