How to carry, seatbag. Small and under teh seat, not behind. I prefer to leave everything I need there (except for the pump). That way I cant forget somethng important.

Tubes (2) patch kit (yes both, one bad patch odf thorns or glass means 2 flats, no patch kit and the next bit of bad luck means walking). A multitool that includes a chainbreaker (with that you can change your bike into a 1 speed even after a fall that has ruined the derallieur). I also like a $10 or $20 in there somewhere.

I woudl say always have water, but hardly need it for 5-10 miles. These days my cell phone also goes in a jersey pocket.

Depending on the trip I might take a lock. I do not count on a lock to stop a thief, but it will stop a grab and go.