I used to have a great bag that I got at Marks & Spencer. It was like a large, soft-sided document case with two internal compartments, one about twice as deep as the other. I used the narrow one for documents and my laptop, and the deeper one for clothes. The best part was that it had two hidden, padded straps on the exterior that allowed me to tip it 45 degrees and convert it into a backpack. So, during the day, it looked like a dignified business case, but then I could flip out the backpack straps, throw it on my back and run for the train, plane, etc. Alas, it died an untimely death ...

Now, I have a 35 litre, black Victorinox backpack. It has a built-in padded laptop sleeve, two very roomy compartments (both large enough to take documents; I use one for clothes and one for documents), and two smaller front pockets. I can certainly fit a change of clothing as well as documents, laptop and other stuff. The padded straps are supercomfortable. It's not as business-like as my old case, but I figure I'm old enough now (42) that it doesn't matter if I carry a backpack to a business meeting.