I use a Lezyne Road Caddy on my carbon fiber bikes, the most cost effective minimalist bag I've found. Hard to beat for the price, and they're durable and easy to strap tightly to the saddle rails with that wide, stout velcro strap. It has just enough room for:
- A spare tube, usually a Continental Race 28 Light.
- CO2 kit, with some kind of tiny inflator doodad.
- Plastic tire lever, don't recall the brand, but it's really tough.
- Spare quick link.
- Lezyne patch kit with two or three kinds of self-adhesive patches. I prefer the Lezyne patches but they're hard to find now, so I carry a few from Park and a couple other makers. The Lezyne kit came in a thin plastic envelope smaller than a credit card and nearly as flat. That goes in the slip pocket in the lid of the Road Caddy.
- Hero Kit multi-tool, the most compact I've found that includes a chain tool, valve wrench, etc. That also goes in the slip pocket in the lid.
- A Mylar packet of DripDrop electrolyte powder, if I remember.
That bulky Planet Bike CO2 kit has moved to my Serfas wedge bag on the steel road bike.
My steel road bike and hybrid wear Serfas Speed Bag wedge bags. Small on the road bike, medium with expandable pouch on the hybrid. Same stuff as above, with bulkier multi-tool, two regular thickness Conti Race 28 or comparable tubes, two tire levers, snacks, sometimes a USB battery for recharging stuff. Sometimes a few first aid doodads -- aspirin, benadryl, asthma pills, bandaids with coagulation pads. These are for longer casual pace rides.
And always a mini-pump on every bike. Usually Topeak RaceRocket, but also a Blackburn Core Slim, in a water bottle cage mount.
Sometimes I'll bungee-cord a spare folding tire under the saddle for longer rides if I'm expecting rougher terrain or lots of broken glass and construction debris. Very common along some semi-rural places with county line h0nky-tonks and McMansion developments. I've slashed a tire on broken slate that was nearly invisible until I hit it. I don't bother with a spare tire if I'm near town, buses and Uber.