Civilian style backpack (Jansport, Mountain Smith, etc), messenger bag (med/small), soft sided briefcase (portfolio or flap closure type), tote bag with zippered top. These are the things that everybody carries downtown and won't stick out. Look for metal hardware or shoulder strap permanently attached to bag to make snatch and run attacks harder.

Carried in the bag:
*Water
*Snack - Granola, whatever.
*Jacket - Windbreaker, rain, etc. Depends on where you are.
*Sun/rain protection - Depends on where you are.
*Roll of quarters and $1 bills for vending machines and phones.
*Extra cash
*Cell phone + charger
*Street map and small compass
*Public transit map
*Small LED flashlight
*Pocket knife/multi-tool
*Lighter - Depends upon where you are
*Pocket first aid kit with moleskin
*Purell/sanitizer
*Whistle
*Self defense - Something to keep a dog or attacker from closing on you. Depends upon where you are