Encase a planet in a giant shield. You attempt to open it somehow and it contains either. . .
Ruins/Precursors/NPR/Automated defences/Automated mines and a store of minerals. Think of it as a random encounter.
Ideally the race trapped within would die out and the planet would have run out of minerals. So it's a dead colony. if there's automated mines though they could have kept mining and shoving the minerals into warehouses for the local alien explorer to load up, or some tech to scavange-- or even colonize it yourself and try to turn the shield back on if there's something nasty heading for you.
Hell, you could make a planet or something surrounded by mines that did pretty much the same thing-- Of course, you'd have to navigate the minefield first.
Of course, if there was a race still alive in there they'd probably be a pretty hostile one after being locked up so long. . .
