After collecting their brand-new bicycle from their LBS, apart from buying a helmet - very sensible - most people feel the need to buy one of those little bags which fit up under the saddle, then wonder what to put in it.
Here's what I do.
For me, it is all part of the activity of cycling to be reasonably self-sufficient on the road. And the most common call is dealing with a flat tyre or minor mechanical.
I have two bags; one for local, more light-weight rides and one for longer trips. So these are just what I do. Others will have other ideas, but it's a place to start.
This is my mini-pack. Okay, it's not black, but when I saw this I had to have it to match the rest of my bike.
One of my peeves is a floppy bag ... following riders whose bag is swinging around behind the saddle grates with me ... I know, I should get a life. So I always cinch it up as tight as I can get it up and under the seat.
What's in the bag?
I wrap most, if not all of my stuff in an old sock. Not only does it keep everything together, it also stops rattling - I hate rattling, floppy bags even more - and you can use it to wipe your hands if they get oily. In this picture you can also see a tyre lever. I can manage with one. You may need two.
And here's what's in the sock;
A spare inner tube: yes you can sit in the cold and rain fixing a tube and wondering if it will hold, but take the damaged tube home and fix it later and get the new tube on as quickly as possible.
A tyre lever: you may need two. Remember, this is only used to get tyres off ...
A patch kit: It is possible to get more than one flat, so it's your last resort. I use self-adhesive ones.
A tyre boot: to temporarily repair serious damage to your tyre. This is a get you home strategy only.
CO2 inflator: These will quickly inflate a tyre to a reasonable pressure. Much faster than using a pump, especially one of those modern mini-pumps.
A multi-tool: I use a very compact one with just a couple of Allen keys and screwdrivers on this sort of ride.
Hand wipes: to clean your oily hands. It's a luxury, I know. The sock helps too.
Some people like to put their cell phone, wallet and stuff in the bag, but I prefer to carry that sort of stuff in my back pocket, or on longer rides, in a handle-bar bag. However, I also tuck in a card with my name, address and emergency number on the basis it will never be necessary and a $10 bill for emergency coffee stops. The bill will also serve as an excellent tyre boot if you're really trying to save weight.
For longer trips I use a larger bag. This one also has an expansion zip in case I need the extra capacity, but if I don't it does a good job of compressing the contents.
This time I'm carrying, in addition to the above:A spare tyre: I actually ride with tubeless tyres and a sealant, but I carry a regular tyre and tube just in case I have a total catastrophe.
A more capable mini-tool: normally I carry one with a chain-tool included.
A Crank Brothers tyre lever: some people don't like these, but it works well for me.
A mini-pump: even if you're using a CO2 inflator, a few strokes from the pump to get some pressure into the tyre before using the CO2 will give you a better finishing pressure.
If you're riding in a group, you can always share out a more comprehensive kit, but the minimum is to be able to fix your own flats.
So there it is. A bit like the mysteries of your mother's purse/handbag. Others will have different ideas as to what's essential and what's not.
But that is what I do.