As far as I can tell there's exactly 3 usable objects (not counting vehicles) in the game: Two guns, and the tank ammo. There's lots and lots of things hanging around which seem to have descriptions and physical presences but can't be operated or picked up. Am I missing something?
As fas a I know, the rest of the stuff is just decorative... But I've seen some videos by SYRSA that show some of them do freaky stuff... but I don't know how to do any of that...
Oh, there are a few different things you can pick up and store. I know you can pick up certain peukins, and there is also a very special item you can find in Dark Keigan which is kinda useless but quite interesting. ... Spoiler: omg spooilrrz It's called a svibra beacon and you can find it in the shadow of the rock that has the entrance to Acrityrda in it. ... Oh yeah, you can pick up those axes/hammers you come across. They don't actually do anything... I think.
Hate to burst your bubble, but svibra beacons litter dark keigan as far as I'm concerned (last time I played it, I saw at least two on the map which to me shall constitute as "litter"). Also, at the end of the day, they're just bright lamps.