Most of us are familiar with the Boy Scout motto, "Be Prepared," but how many actually follow this good advice in all circumstances? The answer is not many. And how many of us carry the so-called "Ten Essentials" every time we go out? Again, the answer is, not many. Just ask yourself this question: "If I go for a day hike and I'm not able to get back to the trailhead by dark, would I be able to survive the night?" Even in temperate climates the answer may be "no." In Colorado where I live , temperatures usually drop dramatically at night, sometimes even below freezing, especially in the mountains. And we all know that rain, even on a hot summer day, can strip the body of its normal warmth and lead to hypothermia.
My advice to you is to be prepared for what is POSSIBLE, not what is PROBABLE. in other words, the forecast may be for clear weather, but you should still carry rain gear because it COULD rain. Obviously there are limits. You don't want to carry a huge pack every time you you go for a jog on the greenbelt. But there are some lightweight things you can carry in a fanny pack without getting bogged down. I would start with the Ten Essentials: food, water, knife, first aid kit, map, extra clothing, fire starters, compass, space blanket, and flashlight.
I also carry at least one 55-gallon plastic bag which folds up pretty small. It makes a dandy bivvy sack if you have to spend the night out. In addition I carry a lightweight plastic poncho which you can buy for less than a dollar to keep my top half and my pack dry. And while we're on the subject of plastic, I always carry two plastic bread bags. I slip them over my socks in wet weather to keep my feet dry. Amazingly, I have hiked many miles on bread bags with no problem!
I also carry a pair of trauma shears which you can buy at Walgreens for cutting up clothing and packs to use as field expedients for bandages or for making a stretcher, and a small hand saw or wire saw to use for cutting down saplings for making a stretcher. These two items are what I call "effectiveness multipliers." They will greatly improve your effectiveness in a wilderness first aid situation.
Lastly, I always carry lights. If you have ever been be-nighted on the trail, you know the psychological value of having even a small light, not to mention the fact that it might keep you from walking off a cliff.
If you are still dubious about my "Be Prepared" advice, go hang out on your favorite hiking trail after dark some evening and see if you think you could survive the night.