A Carradice SQR bag is a more comfortable alternative to a backpack and does not require mounting a rack on your nice road bike. They are not huge, but are large enough for daily commuting, about 1200-1300 cubic inches. I have the Slim model.

I good old '70s-'80s sturdy lugged steel 10 or 12 speed makes a great, tough commuter. I have 3, a Schwinn, a Trek and a Bianchi. The construction is remarkably similar on all three bikes, and all have threaded eyelets for a rear rack.