Weird planets to colonize

I may write more about Civ: Beyond Earth later, but suffice it to say for now that it's a bland and tedious thing not worth the asking price. One of the tacked-on victory conditions is the good old "the planet is quasi-sentient and we must merge our minds with it", just like in Sid Meier's Alpha Centauri. Now this is a perfectly decent staple trope, but it's getting a bit long in the tooth, and I wonder what other Great Secrets a planet being colonized in this sort of game could have.

You know what would be really cool for this kind of sci-fi colonization game? A set of different weird things about the planet to randomly choose from. So when you touch down initially, while you know that there's going to be something weird with the place (otherwise that would be boring), you don't know which one of the options it is. This might add quite a nice amount of tension and replayability to the game. And new planetary weirdness options would make much cooler DLC than some new factions...

With that in mind, what classic planetary weirdnesses can we think of? Here's my list so far:

• The planet is sentient (the good old Alpha Centauri thing).
• The planet is an alien garden and the gardeners will wake up from their nap and prune you if you're not careful.
• Cycles where everything becomes terrible, like in Pitch Black.
• Massive solar activity makes long-range communications and computers extremely hard to do.
• Man-eating shape-shifters a la The Thing.
• The local lifeforms start hybridizing with the colonists, with fun results.
• Telepathic aliens lie buried on the planet, and will haunt the colonists' minds.
• Ruins of a human colony from hundreds of thousands of years ago, which obviously causes some headaches re: history.

Each of those would change the way the game works to some degree, as you'd have to adapt your strategy to deal. Other suggestions?