All my riders have an identical setup.
1 Zefal seat bag from Walmart
2 Specialized tire levers
2 CO2 cartridges, one inflator (they vary)
1 tubes, a pack of Scabs, presta adapter
5mm hex key (I don't use QR skewers)
$1 bill, new.
There's room for my cell and car keys.

For centuries, I add a tube and a boot.
Cell goes in my jersey so I can take photos and call ahead for cold beer.

If I ride the tubulars, I use a Pedro's mini-bag with no tube(s), tucked up in the top tube/seat tube joint. The spare tubular tire is in back.