The bare essentials to take on a bike ride include some sort of identification and a helmet. The i.d. should have your current address on it and it is not a bad idea to also carry a card that tells who to reach in case of emergency. On this card, you can include your blood type, allergies and any other medical condition.
Putting one of these cards in your seat bag in a small zip lock bag and affixing a sticker with this information to the inside of your helmet is a good idea even if you ride with friends. It is possible to get separated from the people you ride with and if you were ever to loose consciousness, you obviously will not be able to communicate this important information.
In Case of a Flat Tire
Getting a flat tire can mean the end of the ride if you are not prepared. You may also have walk more than you care to, to get back home. The things you will need to have in order to fix a flat tire on the road are, a pump, some tire levers, a spare tube and/or a patch kit. Go to this page for a directions on "How to Change a Flat Tire" .
Once you have everything you need to fix a flat on a ride, you will need some place to put it. Three places that you can put these things are, in your jersey pockets (pockets are on the back of cycling jerseys / shirts - if your pump is a mini pump, this will work), inside a seat or handle bar bag, or inside of two water bottles that have been cut up to fit inside one and another (placed in your spare bottle cage).
This last method of carrying you flat changing things (inside of two water bottles) is popular with the racing crowd since it is easy to remove the tools and replace it with a spare bottle when you race. Another flat fixing tool to have is a tire boot in case you tear a hole in your tire. A cut piece of old tire works, but if you don't have a tire boot, you can use a folded up candy wrapper or a folded dollar bill.
Be ready for the weather to change. Getting a forecast is essential for a long ride. Knowing what conditions are coming and being prepared for them will insure that you are always comfortable. Dressing in several light layers is recommended, so that as you get warmer or colder you may adjust your attire more easily. If there is a chance of rain, you should take a jacket that will protect you from the chill of being wet.
Remember that on a bike you travel through the air much faster than walking and wind chill (especially when you are wet) can cause hypothermia.
If you start a ride when it is chilly and you know it is going to warm up, but you don't want to take clothes that you are going to take off in relatively short time, put a newspaper under the front of your jersey. You can take this out, fold it up, and put it in you pocket for the next chilly ride.
Something to Drink / Something to Munch on
A golden rule of riding your bike is to drink before you are thirsty and eat before you are hungry. It takes time for water and food to get into you system. When you are physically exerting yourself it is important to avoid depleting your internal gas tank entirely as you will need some time after you eat to recharge.
Carry something easy to digest in a jersey pocket or a bike bag. There are specialized foods for exercise but fruit or fig-type bars also work well for getting energy quickly.
Drinking water or a lightly sweetened fluid of some type is best done in small increments. A sip every couple of miles helps to insure that you will stay hydrated, reduce your chances of cramping and make sure your motor runs smoothly. Avoid heavily caffeinated or sweetened beverages, as these will speed up the dehydration process.
Being Prepared for Everything
Other things that you can take with you on a ride are: a chain tool, allen keys, wrenches, spoke wrenches, spokes, lubricant, a cell phone, dog repellent, money or credit card. If you are riding in Bordeaux Region of France, make sure you take a corkscrew with you.