Actually, many European women (up to about the age of 50) wear backpacks almost every day. However, they're not the typical "tourist" backpack, but stylish leather backpacks made by top European leather goods manufacturers like Longchamp, Lancel, Delvaux, Louis Vuitton, etc. The "sac a dos" is an extremely popular look and just as stylish as any purse (and sometimes just as expensive: the Delvaux Amitie sac a dos in the cafe leather option costs about $1500; it's a highly desirable item in Belgium). Longchamp makes a very attractive and very serviceable leather backpack for about $200 (prices are much higher in the States) that I see everywhere in Belgium and France. If your coat has an inside pocket (if it doesn't ask a tailor to make you one), keep your wallet there and put everything else in the backpack.
(Brandwise, for under 20s, male and female, the brand is Eastpak; for young women up to about age 25 it's Kipling; for women up to age 50, Lancel or Longchamp, Delvaux, Louis Vuitton, or any other good European designer; over 50s stick with purses, by all the designers above plus the Italians and a LOT of Sonia Rykiel.)

There are any number of styles of skirts/slacks with "secret" pockets for carrying your passport/cards/cash, even a slip with pouches/pockets, available at the various online travel attire/accessory websites. As for your camera, etc., best bet is an ordinary microfibre tote with your maps, brochures, bottle of water visible, with your camera, lens, etc. buried at the bottom.

Any purse, fanny pack, backpack is an obvious target.

If what you are carrying is heavy - and guide book, camera and water bottle are heavy - then a backpack style is easier on your body to carry around all day. I tried a "healthy back" bag and ended up converting it to a backpack. I always use that when I travel now. It's not quite as stylish as the leather "purse style" backpack, which I bought in Italy, that I use at home, but it's large enough to hold everything I need for the day and weighs less. Obviously I keep all credit cards, ATM cards, etc along with the passport in the money belt. I usually also keep my days spending money in my pants pocket. Plus if you want you can use one of the small metal combination luggage locks to hook the two zippers toghether, isn't noticable and acts kind of like a zipper pull.

And by the way, I don't see any reason why women over 50 shouldn't use back backs styles.

Isabel, I'm not dictating that women over 50 avoid backpacks. I'm only noting what I see everyday here in the streets of Brussels and on frequent trips through France, Netherlands, etc. I see very few women, hardly any, in the 50+ age bracket with backpacks, even the designer ones.