If you are looking for a bag that has a bit of structure, try something like the Sherpani Pica.
http://www.ebags.com/product/sherpani/pica/206387
It is 9.06 x 8.46 x 2.95 (inches) and weighs 270 grams. It is a water-resistant woven nylon crossbody bag with adjustable shoulder strap and features flapover construction with a zippered main comparment, several additional zippered compartments, and a side water bottle pocket.
Another is the Mountain Equipment Co-op (MEC) Globetrotter .
http://www.mec.ca/AST/ShopMEC/Packs/Sho ... p?pdtTab=3
It too is a crossbody flapover bag with adjustable shoulder strap, with padded zippered main compartment and additional zippered compartments, but no water bottle pocket. It is 10.62 x 8.66 x 1.96 (inches) and weighs 258 grams. So doesn’t have the depth of the Sherpani Pica, but is padded.
I really liked having a bag for both after my daily walk, and for travelling to and from the Camino. I wanted something that could accommodate a guidebook, journal and pen, tickets, glasses, perhaps a snack, small water bottle, camera, but also had safety features of being crossbody, and flapover with zippered compartments.
Yes there certainly are lighter-weight alternatives such as the little bags that fold up into a key chain pocket or perhaps the Onya Side bag (12 x 14 x 5 inches and 100 grams). But for me, I prefer the structure and security offered by something like the previous two bags I’ve mentioned.