Another great thing you also can always add are ceiling windows and additional structures to this windows. Think about the many openings Episode 1 in Doom had, they all looked beautiful and enhanced the feeling of real 3D alot. Today, with the help of high-end pcs and skyboxes, we can triple-enhance this feeling with just a few simple additional steps.

In screenshot number one you can see a sky-opening in the ceiling out of a organic structure. In the middle of this window, you see a metallic girder, part of the former building. With such windows and girders, you can play a lot with different light levels which also affects the floor. Result: The human eye of a player sees a lot of contrasts, differences and colors and thinks: "Wow, detailz0r!!!" Another great trick in this screenshot was developed by me and I call it, the "fake translucent flat". The part of the room uses 2 skyboxes, a bright one and dark one (can also be done with different colors, not just light-levels itself) and with the help of the VIEWPOINT PICKER I can tell which parts of the girder are completely translucent and which are only half translucent. Nice, isn't it? :) (buildtime for the window 5-10 minutes, buildtime for the special sky effect 5-8 minutes)

The shot two shows off a very advanced way of adding detail to a sky window. The hard part (takes a lot of time) of this are the metal parts which directly connect to the rock behind them, so it looks like a backup preventing the ceiling to collapse. The other metallic thing (the METL1 one with the bolts on it) is just a room-filler, nothing special, just one sector around the ROCK construction. (buildtime 10-15 minutes)

Summary: Adding windows to the ceiling adds always depth to the game and if you use it wisely, you can add lots of detail within the nice architecture of the opening!
Extra-Buildtime: "5-20 minutes" (depends on size)
Build-Difficulty: "Medium - High"