Also, try mixing monsters so not everything is an ice creature with antifreeze for blood and a melting point of room temperature.
Polar variants of animals should count as having cold weather outfits.
A monster that can burrow through the ground can just dig down into the warmer ground when not hunting.
Using tribes of hill giants rather than frost giants allows more foes per encounter. Their fort save is high enough to endure sever cold for many hours by wearing a cold weather outfit.
If the setting is extreme cold, even just a few points of cold resistance still protects against cold environment dangers. Just put that point of cold resistance onto some creatures and BAM, you have foes that can stillsuffer cold damage but won't make fire spells the only choice for the environment.Like say a bloodline of hill giants with just enough frost giant blood to have one or two points of cold resistance.
All Devils and demons both resist cold 10.
Cold Weather Outfit
A cold weather outfit includes a wool coat, linen shirt, wool cap, heavy cloak, thick pants or skirt, and boots. This outfit grants a +5 circumstance bonus on Fortitude saving throws against exposure to cold weather.
In conditions of severe cold or exposure (below 0° F), an unprotected character must make a Fortitude save once every 10 minutes (DC 15, +1 per previous check), taking 1d6 points of nonlethal damage on each failed save. A character who has the Survival skill may receive a bonus on this saving throw and may be able to apply this bonus to other characters as well. Characters wearing winter clothing only need check once per hour for cold and exposure damage.
A character who takes any nonlethal damage from cold or exposure is beset by frostbite or hypothermia (treat her as fatigued). These penalties end when the character recovers the nonlethal damage she took from the cold and exposure.
Extreme cold (below -20° F) deals 1d6 points of lethal damage per minute (no save). In addition, a character must make a Fortitude save (DC 15, +1 per previous check) or take 1d4 points of nonlethal damage. Those wearing metal armor or coming into contact with very cold metal are affected as if by a chill metal spell.