One thing I often kick myself for missing/forgetting in games is to detail to the players how the weather is on an any given day. Not so important when you're dungeoncrawling, but any other time it adds to the immersion and atmosphere of your game world immensely.
Start with a base temperature for your current neck of the campaign woods:
Climate Winter/Spring/Summer/Fall (av. temp in degrees F)
Equatorial 60/70/90/70
Tropical 50/60/80/60
Sub-Temperate 40/50/70/50
Temperate 30/40/70/40
Sub-Arctic 10/20/40/20
These temperatures will range from +10 degrees at 3pm to -10 degrees at 3am.
Then roll 1d20 for any temperature changes that day:
1-10 - Temperature stays the same as yesterday.
11 - Slightly Warmer +5 degrees
12 - Slighly Cooler -5 degrees
13 - Much Warmer +10 degrees
14 - Much Cooler -10 degrees
15 - Hotter +20 degrees
16 - Colder -20 degrees
17 - Scorcher +30 degrees
18 - Cold Snap -30 degrees
19 - Heat Wave +40 degrees
20 - Freezing -40 degrees
Then roll 1d12 for precipitation (adjust by climate, -4 to roll in Arid Regions):
1-2 - No change, same as yesterday.
3-5 - Dry as a Bone, Sunny
6-7 - Light Precip: Icy Mist/Fog, etc
8-9 - Precipitation: Light Rain, Snow, etc
10 - Heavy Precip: Swirling Snow, Pounding Rain
11 - Heavy Storms: Thunder/Lightning w/sleet, hail, heavy snow/rain
12 - Violent Storms: Monsoon/Blizzard/Hurricane conditions (seek cover!)
