There are square panels with round holes - you can stick a fan or grate in them. There's a couple types of crates and barrels - including 'explosive' gas bottles, and a couple types of computer consoles and monitors, ladders, and bulkhead doors, and a whole bunch of other stuff too! There's another panel with a square hole in it, you can put pieces in the hole to make it look different - like a broken floor tile, or a closed hatch, or an opened hatch, or a ladder leading down, etc. Anyway, here's a couple pics :)
Heavy duty bulkhead door!