The pin in my chain tool comes out and I could easily lose it. I always keep the tool and its pin in a sandwich bag.
I have a generic crank removal tool. I tried a small square of steel to keep the plunger from pushing into the threads for the crank bolt. My preference is a short piece of steel rod cut to length so its end protrudes a millimeter or so beyond the threads when inserted in the bolt hole. The diameter of the rod all but fills the hole for the bolt.
If you are confused about whether something, like a pedal, is right hand or left hand thread; just think about whether friction from its natural rotation in use would tighten or loosen the threads. Designers always make machinery so bolts naturally tighten a bit in use. No one wants their stuff falling apart for no good reason.
The binding collar from a rear reflector or a taillight is just enough to keep a slipping seat post from sliding down during a ride. Mount it just above the seat post collar.
Winter clothing with wicking properties is a lot cheaper in a common sporting goods department than at most bicycle supply houses.
Cotton Jersey gloves inside a larger than necessary pair of leather gloves keep hands warm in many winter termperatures.
Plastic food bags over your socks before you put your shoes on hold in a bit of extra warmth for your feet in the winter.
If you lose the instruction sheet or manual for a piece of equipment, it is probably available as a download somewhere on the Internet. Look at some catalogs. You may even see your unit with a different brand name on it. The manual may be available under the different brand name. My Schwinn cyclometer is also marketed as an Ascent. I was able to download a manaul for an Ascent that even looks exactly like my Schwinn manual.
WD-40 on a rag cleans up many greasy things on a bicycle.
An air tank is a great way to inflate bicycle tires. Just pump the tank up the the required pressure and there is no guessing about when the tire is at the proper pressure. Most air tanks have a pressure gauge on them.
For extra safety at night, cut some reflective tape to fit parts of your helmet and apply it. Some reflective tape is silver, not just red. (Some helmets claim adhesives could change the chemical composition of the protective material, but mine has not changed.)
Put a label on your helmet or your bike that gives your name, address, phone, blood type, and a contact number in case you are knocked unconscious in a crash.
When a bare brake cable flutters against the frame it makes a distracting noise. Get a piece of automotive vacuum hose an inch long. Slice it lengthwise in a spiral pattern. Wrap it around the cable near the middle of its run and tape the hose to the frame tube.