Space Engineers is, in the words of its developer, "a sandbox game about engineering, construction, exploration and survival in space and on planets." You can build space stations, planetary outposts, and pilot ships.
As well as a 50% off sale on Steam this weekend, the game will also be free to play. Check your Steam library and it'll be sitting in there waiting for you. It's an Early Access game, but packed with features, including the recent addition of moddable, destructible planets. You can read all about that here.
