Possibly the smallest game I own, but it packs a bigger punch than that.
Solar 2's main strength is that you can play at your own pace, with your own goals, and that leads to a good few hours that you can spend just bumbling around in space and figuring out what you fancy doing. Going from a tiny asteroid to a black hole? Staying as a solo planet and building up a small armada of ships? Just flying around like a lunatic smashing into stuff and dying repeatedly? All viable playstyles.
If you want more structure, the story and challenge missions offer a good balance between fun and difficulty, and add considerable longevity to the game. The general idea is to work your way up the planetary body hierarchy - from diminutive asteroid through to planets, stars, star systems and eventually nature's biggest vaccuum cleaner. There are individual missions for each stage of being, some of them simple, others very difficult. I have yet to witness another game that lets you play as the stage itself at a rock concert.
For a little indie title, there's a surprising amount to do, and while it could perhaps get a little samey after beating everything the game has to offer, there are physics options that you can use to craft a more creative playing experience - like mass space dogfights between hundreds of ships, for instance.
Or, you could just let the cool ambience of the game float you away on a paradoxical vaccuum breeze...