For the few times I go without panniers, like grocery shopping, which is less than 2 miles away, I use something called a "Campus Pack" bought at Cabelas last time I was there. The shoulder straps are wonderfully thick and padded so they don't dig in, have lots of extra length for adjustment to accomodate the bent over position of a drop bar bike, and the sternum strap keeps them stable and pulled in, even for those of us with a large upper body. It has two biggish main compartments, the smaller of which holds my thinkpad comfortably. I prefer it to panniers if carrying a computer, as the computer is better isolated from road shock by being on me rather than the rear rack.

http://www.cabelas.com/cabelas/en/te...results1.jhtml