I haven't played anything like enough of sandbox building/exploration game Space Engineers [official site], but the recent addition of explorable planets looks like a solid gold reason to. They arrived back in November, but this new video makes a strong case for why they demand more attention.
For starters, these things are big enough that it'll take something like an hour to drive across them - which means you're not exactly going to run out of space to build stuff. But getting to that point involves landing on the planet first, and that's a process which involves building three different types of vehicle. I like the idea of a planetary landing being a big deal, not the magic button-push of something like Mass Effect. There's also some lovely real world-inspired science, such as space stations crashing through the atmosphere because they spent too long on the dark side of a planet and so ran out of solar power, as happened to poor old Philae.
Here's one of the devs, who appears to work in giant glass future-palace, talking about how it all works, how the game's planets get made and showing off some lovely, LEGO-does-3D-Proteus footage too.