(source: thingsorganizedneatly.tumblr.com) As a lady who indulges in the carry-a-bag-anywhere, it's possible to cheat, although my bag is not large. My last deliberately planned "go bag" was a small duffle in my boot that contained one entire krav-teaching outfit—school logoed pants and shirt, boxing wraps, sports bra...in theory this would also cover a sudden change of clothes or need to overnight. Did not include toiletries, though.
Right now, in my knapsack (leather, small, pretending to be a girl) I don't leave home without, aside from keys and cash, USB cables for phone, tablet, iPhone, a charging plug, headphones, two widths of black pen, mini sketchbook, tampons, panty liners, folding dress knife (not practical for fighting, since there's no hilt—see below) and a day's worth of my prescription meds, above and beyond the normal stuff I'd carry with.
I was chatting with someone the other day who had THE WORLD in his knapsack that he went nowhere without. His knife had a five inch blade, he had enough rope to rappel from the tenth floor, a mad amount of computer processing power and at least 1TB in storage—and the hand cranked charger to power it all. Either he's your best friend when the zombie apocalypse happens, or he is going to kill and cure you for good eatings. It's so hard to tell with some people, you know?
Me, I want to beef up my go bag (car only) back to a change of casual clothes, bottled water, a wrap that doubles as a blanket, and a week's worth of toiletries and beef up my car's first aid kit with more pain meds.