I just wanted to take time out to show you all something I've been working on. I call this tote the BOSS. It would work in a number of situations...it can go from baby bag to beach tote to carry-on bag without missing a beat.
The prototype shown I made for myself as a school bag. I can carry my sketch book, my camera, pens and pencils and whatever else I need for my day. It's even roomy enough to put my purse in, in case I don't feel like carrying everything and the kitchen sink with me when I want to go grab lunch. The shoulder strap area is such that it may comfortably fit on one's arm, even if they were wearing a coat. In the prototype I made two welt pockets...the final version will have four. They can hold a multitude of things that you need to get to pretty quickly like your MP3 player, cell phone, metro pass, etc. These pockets are just deep enough to also hold a standard bottle of water, soda, even baby bottles!
Inside there is a spacious zippered pocket, a keyholder strap, a large patch pocket (just right for holding a sketchpad or a magazine), and a gusseted pocket (I use mine to hold my camera and battery charger). On top of all that, the bag itself is plenty roomy...just look at the pics!
Since this bag is made to put in work, I think I will stick to fabrics like duckcloth canvas, heavy denim, and maybe upholstery fabrics because they are so durable. My favorite marine vinyl wold work also, but I'm thinking it would be a bear to work with when it comes to the strap.