Outside building

[ZyLogicX]

That doesn't mean it's impossible, it just take a bit more time.

When making grass, (Found in static_objects>outside) you have to make sure that 'collides' is unchecked otherwise the player keeps coliding with the grass (since its technically a static object)

Skybox:
What really defines 'outside environments' from indoor is the option to use a skybox, This will really changes the players perspective and it just feels good. You can do this by opening the leveleditor, choose 'edit' next to 'file' and select 'level settings'. Now mark the 'active' box and browse for a skybox, which can be found on various websites... LaCaza also has a skybox, so you may want to experiment with that texture. It is optional to give your skybox a colour, but personally you should only use dark colors if needed.

Entities:
Debris is probably what is best... especially when you put collapsed walls in your map.

Particle Systems:
This is optional, but it creates a very outside feel. Go to the particle tool and browse to 'Particles'
Scroll down untill you find "Ps_water_rain.ps"

Hope this helps, even when it's only a tiny area, this will clearly improve the vibe...