Go to your local sporting goods or discount store and buy a handful of cheap rain ponchos. My local store sells them for 88 cents each. Toss a couple in your main camera bag, (one for you, one to drape over your camera) Put another one in the glovebox of your car, and every other camera bag, or box of gear you have. Sometimes I tape them to the inside of the box lid to keep them out of the way, yet easy to find.
All kinds of liquid based thing are now being sold in "pen" and "moist towelette" form. This packaging is small and rugged, making it easy to carry in your gear bags. Some of the most helpful I've seen include stain remover, suntan lotion, machine oil, insect repellent, and hand sanitizer. I'm sure there are more. It's often the little things that can save the day, so go out and grab several of these and add them to your camera cases, messenger bags, or glovebox.
I was asked today what was the most important accessory to have as a professional photographer. They were expecting an answer like external flash, or tripod. My answer - a lens cloth. No particular brand, or even size. I have a random collection that I keep stashed all around. In the armrest of my jeep, every camera bag, laptop bag, inside the zippered pocket of my camera strap, and pretty much anywhere else I might find nearby when using a camera. Most of the time I have a small one tucked into my pocket, and I hate carrying a lot of things in my pocket.
This is my hands down pick for the one item you should always have with. you. Is there any such thing as a lens that is too clean? They are small and cheap, so you have no excuse not to have one handy.
Everybody knows you should keep a spare $20, $50, or even $100 bill tucked away in a back pocket of your wallet for emergencies. Problems where do you keep it where you won't see it and be tempted to spend it, or when you're like me and use a money clip. The solution is to go to a local drugstore or on Amazon and pick up a small aluminum keychain pill holder. I bought a set of 6 on Amazon for $7. They measure 1 7/16" X 33/64", and use an o-ring, claiming to be watertight. Take your emergency cash, keep folding it in half until i fits in the holder. Now anytime I'm out, I've got an emergency stash, hopefully enough to get me out of whatever jam I managed to get