Sharing the kit I keep under my bike seat to help others make similar. Covers common needs from flats to medical to picnics. Crafted from experience, comes to someone's rescue often (but don't ask me to change your tire).
That's (L>R T>B) spare tube, Everclear, bandage/alchwipe/AAAs/$, tire levers, pocketknife (w/ corkscrew/tweezer/...), wrenches, multitool, CO2 and adaptor, patch kit, chain tool, spoke tool, naproxen/pseudophed/albuterol/....
The wrenches cover common metric and English sizes optimized for compactness, had to find them online. Finally, there's some spare electrical tape wrapped on a handlebar. The most common situation I can't help is a second flat unless someone else has a pump.
I also almost always carry a small bluetooth handlebar sound system with its own cargo space, allowing me to also carry on-the-go charging capability for all phone types, rearview shades, a snack and stickers to share.
There's other good threads on the topic reaching similar conclusions: