First, a little about me as a cyclist to properly set expectations here: I've been an regular indoor cyclist since 2018, but now that I work at Zwift, I usually do one 14-mile outdoor group ride once a week with my coworkers (which is awesome) with some occasional solo rides thrown in every once in awhile. I ride a endurance road bike with inner tubes (although might go tubeless one of these days). I'm on the high end of amateur at best, but I spent a decent amount of time researching what to carry in my saddle bag and have incessantly picked the brains of my coworkers who are much more serious cyclists than me.
So, here's what I carry when I'm out riding!
Silca Mattone Seat Pack
I wanted a small, low-profile saddle bag that was large enough to carry just the essentials, noting that my longest rides thus far are around 20 miles (and I'm willing to call an Uber or my wife if I have a more serious issue). The Silca Mattone Seat Pack was just big enough to fit the things I wanted to carry, and the BOA closure cinches up nice and tight so the bag doesn't move at all while I'm riding. It's also waterproof, but that wasn't a huge factor for me because I'm very much a fair weather rider.
Topeak Mini 20 Pro Mini Tool
Ok, with 23 tools, this is probably a bit overkill for the types of roadside repairs that I'm likely to see myself (let alone be capable of dealing with without first consulting a YouTube video). But, the Topeak Mini 20 Pro had great reviews, very clever design, a small and flat form factor (which was the perfect size for my saddle bag), and contained the tools I was most interested in carrying: basic Allen wrenches in many sizes, a chain tool (which lets you open up a chain by forcing out one of its pins, and then re-assemble the chain by forcing the pin back in once you've repaired it), and a chain hook (which keeps your broken chain held together while you repair a link - great for if you're riding solo).
Topeak makes a bunch of mini tools in various sizes, so you can get more or less depending on your space constraints.
Tubolito TPU Tube
I ride a road bike with inner tubes, so I decided to carry one spare tube just in case I had a puncture that I wasn't able to repair (more on tube repairs later). Sure, I have two wheels on my bike, so I'm banking that I'll only ever have one un-repairable flat tire on any given ride – otherwise, I'm calling for help.
Even though they cost significantly more, I decided on a TPU tube from Tubolito rather than a much cheaper butyl or latex tube simply because TPU tubes take up significantly less space (seriously, check out the size difference). I've seen a few examples of folks damaging their TPU tubes when inflating them in the same way they'd inflate a butyl tube, so it's worth checking out an example mounting video before using these for the first time.
Some folks I know who ride tubeless still carry a spare tube as well: just in case they have a flat that's severe enough that it can't self-seal, they could throw a tube in the tire and use that to limp back home.
I carry my spare tube in a Ziploc bag within my saddle bag, both to protect the tube from potential rubbing against other items in my saddle bag, and because the Ziplog bag itself may be handy in the event of a roadside repair.
CO2 Canisters (2x)
Lezyne Control Drive CO2 Bicycle Tire Inflator
In the event of a flat tire, I decided to carry two CO2 canisters instead of carrying a small bike pump mainly because of their smaller size (and I didn't want to give up one of my two water bottle mounts to mount a bike pump). I went with the Lezyne Control Drive CO2 Bicycle Tire Inflator because of its small form factor and positive reviews, and it came with one CO2 cartridge and a neoprene CO2 cartridge sleeve (which is great, because CO2 cartridges can get pretty chilly when you're inflating a tire).
Now, let's address the math problem here: My bike has two tires, but I'm carrying one spare tube and two CO2 cartridges. As I explained earlier, my rationale with carrying only one spare tube is to minimize the size of my saddle bag while betting that I'll only have one un-repairable punctured tube during a ride. But, in the event that my other tire gets a flat as well, I'm betting on it being repairable, so the second CO2 cartridge is for re-inflating the repaired tube. Time will eventually inform me as to whether I've made the right decisions here.
Park Tool GP-2 Pre-Glued Super Patch Puncture Repair Kit (6x)
For a small tube puncture, I have Park Tool GP2 Pre-Glued Super Patch Puncture Repair Kits in my saddle bag. These things are tiny and thin - about the size of a US postage stamp - and even though I probably only need one or two in my saddle bag at any given time, I opted to carry the whole pack of six because I had the space (and hey, maybe I'll be able to help out others while I'm on the road). Since these are pre-glued, you can just prep the damaged tube using the included sandpaper, peel the patch off of its backing, slap it on, and you're (probably) good to go.
Park Tool TB-2 Emergency Tire Boot
In the event that my tire gets seriously damaged, I carry one Park Tool TB-2 Emergency Tire Boot, which is a thin membrane with adhesive on one side that you can stick to the inside of your tire so that your tube doesn't extrude from a gash. This is small and thin – smaller than a credit card – and took up almost no space in my saddle bag. That being said, a lot of more seasoned cyclists I know said that they've just used a folded-up dollar bill to patch up tire gashes, so this may be overkill for most situations.
SILCA Tire Levers Premio (2x)
I know some folks can remove and replace tires without tire levers, but I nonetheless respect the benefit of the right leverage and the proper application of strength. These Silca Tire Levers Premio are small, look sharp (because I mean, what are you even doing if you aren't showing off your tire levers when repairing a flat), and I liked that they have nylon contact points so as to not damage carbon fiber wheels (which I don't have, but hey, someday!).
SHIMANO 11 Speed Chain Quick Link Connector
In the event that my chain breaks while out on a ride, I carry one set of SHIMANO 11 Speed Chain Quick Link Connectors, so I can use my multi-tool (covered earlier) to remove the broken link and replace it with this quick link.
These quick links are so small that I just tuck them inside my multi-tool, which takes up slightly less space but also means I'm less likely to have to hunt for them in my saddle bag.
You'll have to get appropriate quick links for your type of chain - in my case, I'm running a SHIMANO 11-speed chain, but similar products are available for other chain manufacturers.
Latex Gloves (2x)
Ok, nothing fancy here: latex gloves take up almost no space, and I figured there are plenty of situations where I might want to have some protective coverings over my hands during a roadside repair, like if I'm handling a dirty chain or dealing with bodily fluids from an injury. I also felt like these gloves would help minimize any rattling of the items inside my saddle bag, but it's packed tightly enough that this likely doesn't matter much.
Spare Contact Lenses (2x)
I'm pretty nearsighted, and if I was to lose a contact lens (or worse, both) during a ride, I wouldn't be able to safely continue riding. I always carry one extra set of contact lenses in my saddle bag, just in case.
What I'm not carrying yet (but should)
Although I think I have most of the essentials for dealing with basic roadside repairs, there are a few things I'm not yet carrying but definitely should.
First Aid: Even though I'm low on space, I think I could easily include a few band-aids, a piece of gauze and a few strips of tape, and an antiseptic wipe – these would be great to have in the event of a simple injury.
Cash: I always ride with my phone and wallet, so I'm not super worried about not being able to pay for something while out on the road (e.g. food, snacks, Uber, etc.), but it'd be smart to have a small amount of cash in my saddle bag just in case I forgot my wallet and needed some money in a pinch. Plus, I could finally join that club of cool people who use dollar bills to repair gashes in their tires.