One will compactly hold a few hex keys, your insurance card, a business card and a little folded money. If one of your cards is laminated, it will help everything else stay in order. Foam pipe insulation is cheap, light, easily cut to size and makes a fine cushion between your bike frame and any bike carrier. Slip a sock over reach end. If you turned to look back, you need a mirror.
Different types attach to eyeglasses, helmets, brake hoods or handlebar ends.
Try one, and you' ll wonder how you rode without it. Carry a pen and some business cards. If you borrow water at a church or other private property, leave a short note of thanks. You' ll make a friend for yourself and future bicycling visitors. On multiple-day tours, souvenir shirts shirts and other acquisitions can add significantly to your luggage.