Yes, use sunlight. Unless it is nighttime. Well, even then you can have moonlight - it is never 100% dark at night.
Different light tools have different implementations of this.
In Bengt Jardrup's (aka AguirRe's) light tool and its descendants, you put certain keys into worldspawn, the most important one being _sunlight (this is pasted from one of my maps):
"_sunlight" "255"
"_sunlight3" "60"
"_sun_mangle" "130 -60"
"sky" "purple_"
"_sunlight_color" "1 0.8 1"
I believe you can only have the colour value in newer versions (MH's? BSP2 version? not sure). _sky loads a skybox; make sure sun_mangle is set in accordance with the actual sun in the skybox. You don't want the sun in the east with the sunlight coming from the west...
In hmap2, place a point light to serve as the sun (or several), target it at an info_notnull (? I forget, it's been awhile) and then IIRC there is a key or something that tells the light tool to treat it as sunlight. It's in the documentation, sorry I don't use hmap2 at the moment.
Edit; the bit about functional lighting still applies, even in an outdoor setting. Light should still guide the player. See here:
http://media.moddb.com/images/mods/1/15 ... -04-13.pnghttp://img119.imageshack.us/img119/786/pripyat38za5.jpghttp://guidesmedia.ign.com/guides/48046 ... pyat_3.jpghttp://imageshack.us/photo/my-images/51 ... t20al1.jpghttp://www.cheats.ru/uploaded/e/enemyte ... s02254.jpgNot always easy to do. If you can't use light to mark important spots, then you'll have to make do with ambient sounds, silhouette, or the good old minimap.
I'll add this to the other post I guess.