We actually kept and still use our Baby Gap knapsack / diaperbag.
It doesn't look like a diaper bag at all - it's plain black, with a teeny-tiny teddy bear logo on the middle of the flap and the words "Baby Gap". Very discreet.
But it's extremely light, very durable and hard wearing (ours is six years old and has been loaned out for extended periods, twice), with TONS OF POCKETS. There is a zippered flap pocket, a front deeper pocket, two side pockets (for bottles of ANY type, ha ha), and then the inside has a split mesh pocket along the front, and then a double-slotted pocket, a bit deeper, along the back.
Most importantly, the shoulder straps are hugely wide, so it's comfortable for wearing all day long.
I've bought it for two other girlfriends who swear by it.