It's always very hard to design outdoor areas because you always have the aim to make them look as natural as possible, as never ever someone has changed something to the landscape. But in many maps' outdoor areas - (thx goes to experience™) - I had never the feeling, that I was somewhere outside in the wilderness. Mostly I though of another UAC courtyard. So how do I try to achieve a realistic outdoor area?! Well, almost as everyone else, I use the DoomBuilder sector drawer, create the basic shapes of my area, add a wall texture and a floor texture, make the ceiling F_SKY1 and well... then I am almost finished, just the detail is left ;) and that's the essential part of the production!
In screenshot number one you see one of the many rocks, I add to the center parts of my large outdoor areas. To simplify it, it's just a basic stone-shape with different heights and on its top, some grass is growing, not to forget the HUGE BROWN TREE on the top of this all ;) The important thing with this is also the choice of colors, which always depends on which planet or place you are at the moment. But try to use just 2 different colors and one of them always brown (rest can vary between gray, green and a very light brown like the one from BROWN1).
Ok, so far, we have a rock now with a tree placed in some parts of our outdoor area, but can this be all? Screenshot 2 shows another very good trick to emulate a real outdoor area - beyond the fact, that yo will have to put several differen "rock-stairs" in the style of shot 1 directly to the walls of your outside area to make it look much more real and not only like a "1-sector" canyon. What you really need in outdoor areas are different floor-flats and a few small height-differences of this floors (as you can see the one in shot 2 , GRASS). Well, have you ever stood in front of a natural grown meadow? I don't talk about that 1,2cm english meadow, I mean a real grassland in the open countryside. If you look close, you also see that some grass-surfaces have different colors as others. On some places you can see small trails from animals or humans and this is exactly what you have to add to your outdoor areas to make it look realer. Oh well... and a good sideeffect of this is the growing level of detail ;)
Summary: The "Outdoor Area" Formula: One takes some linedefs which vary in length and angle, adds outside textures and flats in two colors (one brown), adds some stones (with border) and trees to the center- and border-scenery, adds some different floor-surfaces, adds a trail and finished is the outside area ;)
Extra-Buildtime: "30-45 minutes" (depending on size)