I've definitely acquired my fair share of accessories over the five years since I bought my first road bike. And I was definitely overwhelmed when I left the store with that purchase, not bothering to buy anything extra and thinking I had everything I needed in the bike alone. Boy was I wrong--I couldn't ride this sucker like I rode my Costco special I've affectionately named "Big Green" that got me the places I needed to go and even tolerated 20-30 miles on the Lakefront Path before telling my knees no more. Once I upgraded, I learned that I'd need more than a water bottle (if I even remembered to bring that) with me on the trail. So when an athlete I coach asked me about bike gear to buy, I sent her this list of the basic bike gear--mostly what to shove into your saddle bag--I wished I had purchased at the get-go rather than days before a race. I called it "all the stuff to help with a flat or problem on the road."
Bike pump. Definitely key, especially a floor pump. You'll see people toting these around in transition area and a lot of people bring their own for fear of not being able to borrow one or getting a flat on the way to the race even though you pumped them full of air before leaving home (I got a flat with my bike in the trunk of my car once). And it's good to have at home to pump air in your tires before going out for a ride. Village Cycle just up the street from me has air outside the store...kinda like the way you go to a gas station to get air in the tires. I feel like such an idiot for not buying a floor pump until my road bike was a year old and I had already raced my first triathlon--I relied on a very basic, very cheap hand pump--and I learned how crucial that pressure gauge was. Finally, finally I wouldn't have to guess on my tire pressure, or have a tube burst during a ride (luckily my husband's and not mine).
Mini bike pump. Consider this an optional item but some people choose to carry a mini pump when they're out riding so that they don't have to fumble with CO2 cartridges (I'll get to those). The mini pump is nice because you can often attach it to the downtube on your bike and always have it with you in case you're out riding and notice that your tires need air. Or if you're racing, use the CO2 cartridge to fill the new tube, and then screw up and lose the CO2 into the air rather than in the tube, you have another method of getting the tube to fill--that happened to me last summer where I got a pinched flat and the tube wouldn't inflate (turned out it was punctured somewhere). We wasted two CO2 cartridges to fix it and then used a new tube and needed a hand pump to fill it. And if you can find a mini pump with a pressure gauge, even better.
Tool kit, patch kit and tire levers. These tools are the basics to carry for flat trouble and basic repairs. You can use the patch kit to patch up a tube--or so I've heard but have yet to use, ha ha. But sometimes you're better off swapping tubes on the road and doing the patching at home later. The tire levers help you get the tire off the rim of your wheel. Some people can barehand it--not me--but these are usually most helpful especially when trying to get the tire/tube back onto the wheel. The tool kit is good to have in case your seat needs adjusting, something feels loose, etc. And wiith the different screwdrivers and allen keys, it also comes in handy for building ikea furniture or having in the car on a camping trip (I've used mine for that more than on the road).
CO2 cartridges. I had no idea what these were used for the first time I had to write a list of gear to bring to a triathlon. But I quickly learned how handy CO2 cartridges are for races: They inflate your tire super fast after you change a flat. And each canister holds just enough to get the tire plenty plump so you don't need a gauge to check the tire pressure. The confusing part lies in buying threaded or non-threaded cartridges--easily solved by having an inflator that uses both.
Tire inflator. You can't just buy the cartridges, you need a little device to use them, the inflator. Last summer a group of cyclists told me that this was the good kind to have because of the trigger. Wish I had known that before I bought the simpler version that leaves more room for error and wasted cartridges.
Spare tubes. Feel free to laugh at my stupidity for thinking I'd have to go to the bike shop every time my tires went flat. Tubes to the rescue--and they are so easy to carry and always have at least one on hand. Now I don't leave home without one, and they've saved me from walking 10 miles home on more than one occasion. Tthe tubes are folded together into a tight package that easily slip into a bike bag. Some people even toss them in the back pocket of their bike shirt but to each his own.
Bike bag. So with all of these goodies, you can squeeze most of them into a saddle bag to have with you on your rides. You'll never notice it's there and then you're prepared in case of a problem. But finding the bike bag itself can be a challenge; I had issues finding one that would fit my tri bike.
I think I've covered the basics above, or at least what I've been toting around. Granted I still feel like I failed my athlete by leaving her in the dark before triathlon camp over the weekend. I was psyched that she'd learn a ton about the sport in a short time period, which she did, but I failed to tell her some basics before she left. Oops. But question is: did I miss anything?