If you're looking for a casual bag, I highly recommend the Jansport laptop sleeve backpack. It's thin, lightweight but sturdy and really easy to carry. I put my MBA in the main padded (but not bulky) compartment, an iPad or iPad mini in a separate full-size pocket (big enough for documents) and any cables/chargers/accessories in the front zip pocket.
I'm not sure it's that size and it's not exactly the one as pictures, but as I said, I bought it five years ago. My 13 inch MacBook fit perfect and the same goes for my 13 inch MacBook Air. Inside, it has an added velcro belt to put in the computer more securely. You can put your iPad on top of your computer as well, it's certainly deep enough. The pouch has a few separations for cables. I sometimes put a book there as well as the cables, or my iPad. I'm very happy with it.