1. Spare Tube
2. Patch Kit
In addition to a spare tube, you'll want to carry a patch kit as well. But isn't that redundant, you ask, when you're already carrying a tube? Not really. Murphy's Law means that you'll get a second flat in the new tube just as soon as you've replaced it. Plus, you are really carrying these items to be able to help cyclists who might need it, just as much as for your own good, right?
"The patch kit [I carry] is for other riders that my need help," says Pennsylvania cyclist Brad Morris. "Luckily I have used the patch kit 6 times, while still need to use the tube."
Plus, patch kits are typically quite small and are a nice "one-size-fits-all" insurance policy against tire problems.
3. Tire Levers
Whether you carry a patch kit or spare tube, if your tire goes flat, you will need to find a way to get air back into it. That's where a nice little pump comes in. Usually clamped to your frame, these mighty little dudes will put enough air in your tire to get you back on your way.
Some riders prefer to carry CO2 cartridges - little battery-sized cylinders that deliver a burst of pressurized gas and refill tubes in a fraction of a second. They are lighter, but require a bit of practice to use, else you can blow out the tube you've just replaced. Plus, they cost about a dollar a piece, for what is typically a one time use.
For any number of potential fixes or adjustments you might face on the road, a multi-tool is a handy gadget that you'll want to take along no matter how short or long your ride. A multi-tool typically comes equipped with a dozen or more individual tools in various sizes, including Allen wrenches, hex bolt wrenches, screw drivers, a chain tool and more. Tucked neatly into one small package, it's like a portable tool box for fixing your bike - in more ways than you can ever imagine. Plus, many come outfitted with a bottle opener too, when the situation becomes particularly severe.
6. Cell Phone
How did we ever get by in the days before cell phones? For calling home to get picked up in case of breakdown, for calling your buddies who may be ahead of you or behind you on the route during those longer rides, or for just calling ahead to order a pizza at your favorite place, there is no reason not to carry a cell phone if you've got one.
There's at least one rider I know of for whom a mobile phone is all the equipment and tools needed on a ride. In case of a breakdown, he calls the bike shop (free service with purchase of a bike) to come get his wheels and then rings a cab company to send him a taxi for a ride home.
7. Identification/Money/Insurance Card
This is one of those just-in-case items that you bring and hopefully never use. Take along a few dollars for drinks and snacks along the way. And, in case of a split in your tire, a dollar bill can be laid along the split inside your tire to keep your tube from bulging out too badly until you can get it repaired. And, be sure to bring along copies of your identification and insurance cards. God forbid you get into an accident, but in case you do, you'll definitely want and need these items. Tip: on the backside of these documents, write down your list of emergency contacts as well as any special medical instructions or allegies to medicine you might have.