I always carry the following(no matter the bag size): Wallet, phone, change purse, pocket Moleskin, two ink pens(A nice one and a cheap one to loan out), emergency medicine, saline spray, mints, lip balm, lotion, stain-cleaning wipes, handkerchief, plastic shopping tote(the kind that folds down and has a clip), sunglasses, tissues, pepper-spray and a small multi-tool.
What I *like* to have when I am carrying a larger bag: Personal-sized Filoday diary, water bottle, small umbrella, make-up bag, a novel, a tie or silk scarf(if I am not already wearing one, or a formal one for emergencies), phone power-bank.