Keeping Levels Visually Interesting Outdoors

Right now I'm working on a level that takes place in a mars quarry. I can make indoor areas interesting, But outdoor ones look bland. Does anyone have tips for this? Thanks!

(Images are in separate levels)

Well, you have yourself quite a decent inspiration in the location you chose: A quarry. Check out how quarries look. Your walls look rather straight and a real quarry has walls that have gradually been chipped away. Also some „equipment“ is always a good idea. Next you could also think about the age of the quarry: Is it new or abandoned ages ago (and thus overgrown with vegetation or featuring other abandoned buildings / machinery. 

If you're mapping in UMDF format I can suggest making rock piles with random sloped surfaces and dot them around the place. They break up the totally flat ground look, add a little bit of realism and can be useful for setting up play points (ie: cover, hiding monsters, ways to higher areas etc).

Realm667 has a lot of outdoor vegetation and environmental decorations you can use too.

I wouldn't use sky texture, at least not so low, it limits you and the fact that it's level makes it look all the more artificial. IMO try to make indoors look like outdoors. Fake it. Add small details -- they make the map alive. Plus, make it BIG, so outdoors aren't just hallways with sky texture.

outdoor parts "come alive", by way of height variation, and there are plenty maps to look at. Don't just spam the same texture everywhere, because that looks bland. How colourful you wanna go is up to you, but "mono-texture outdoors" will always look "fake" no matter how many  slopes and lighting gradients you slam in there.

Something I made a while ago, uses only Doom1 stock textures except for the lava-fall in the distance, format is "limit removing Doom1":

As everybody else already has explained - us height variations and decorations and make the outside areas big. My favourite way of using outsides is to use them as a sort of hub from which you will have access to different parts of the levels. I also like it a lot when you can watch the outside from inside the level, so I use a lot of windows (which also is great to help the player orientation-wise).




 

I'll bite. Here's a screenshot of a *thing* I am currently working on:

Notice a few things about it. Simplicity is always a factor when building large, open outdoor areas. Note that only two textures dominate the majority of the scene: SP_ROCK1 and ZIMMER8. While they're of a similar color, their details contrast quite nicely, which allows the ZIMMER8 formations to draw the player's eye to the major points of interest in the scene: the cave entrance to the lower right, the far cave with the brick building, and the cliff-side steps climbing along the edge of the pit.

Notice also how I've alternated between two different flats at different height levels: MFLR8_3 and FLAT5_8. This allows you to very clearly distinguish the different height levels of your play area. It's probably also worth mentioning that this scene started off as a flat expanse, and I created the height levels by sinking a large portion of that expanse into the ground and simply building a rock staircase sweeping around the edge. It's very simple and smooth to navigate, really acting as any "indoor" structure might, while still maintaining its sense of place as a canyon.

Finally, pay close attention to the geometry of your outdoor formations. It's easy to just draw a bunch of jagged lines and call it a day, but with a little finesse you can give your cliffs and cave walls a little more character. Depending on your limitations, of course (here I am using a limit removing port). Take note in particular of the ZIMMER8 cave in the right half of the scene -- it has two rounded protrusions that serve to mark the cave entrance, while the sides jut out at a sharp rectangular angle (it's kinda hard to see that here). One other "quick win" you can try out is darkening the hard corners of your outdoor walls as well, which gives the scene a little bit of lighting contrast and can help things from appearing too flat.

Anyways, this is just a bit of stream of consciousness, and all things I consider while working on my own maps. I hope it's helpful to you. 

I generally find it best to add features in all the places outside of the play area.  Here's a "before and after":

Just a few extra sectors at different heights and light levels makes things look much more interesting.