First, carbon. You already know you don’t need it for your casual weekend pursuits, because you’re not counting ounces the way a professional road racer would. But to quote an article from Velonews, “Remember that professional athletes operate in an entirely different environment than the rest of us. They are all very close to each other in terms of fitness, and they are also all very close to being the absolute best a human being can be. In short, you’re much better off upgrading your legs and dropping body fat through proper training and diet.”
Keychain style flashlight or headlamp, LED type, not incandescent: The keychain style is tiny and lightweight, the headlamp style is a bit bulkier but easier to use. You can get these for a couple of bucks or less at the local big box hardware store. The LED ones are lighter, more durable and last considerably longer. Very useful if you need to make a repair after dark! I keep a keychain style one as a backup as I normally have a headlamp in my backpack.
I brought my bike into Kyle's shop to fix the noise in my rear wheel. The mechanic suggested that I need to overhauled the rear hub for $30. The next day the noise came back even worst. I brought it back to the shop and they said it was the freewheel driver not the hub that needed to be work on and it would cost me extra $30 to overhauled it again plus the part to fix it. At this point I walked out of the shop and took my bike to David's world. The mechanic at David's world took a quick look at the rear hub and indicated that the axle was put back too tight when Kyle's mechanics were working on it and further identify the issue. I ended up paying David's world the $30 plus part to fix my bike, but the major different is that David's World has 30 Days services warranty claim. If you have any issue with your bike within the 30 days the service was performed you can bring it back and they will fix it no question ask.
Maintenance is the best prevention. “Prevention is the best medicine, and if you’re taking care of your equipment it’s super key. Bikes don’t just fail. If they’re in really good working condition and you clean them at night, and you’re religious about it, bikes are kind of like cars — they’ll start talking to you if something is getting worn or getting creaky or needs to be replaced.”