Thule Subterra Convertible Carry-on

I picked this one for how it switches between a backpack and a duffel-style carry, which is the kind of flexibility I look for in a 40L travel bag. The removable laptop attache and the way it opens flat are the details that matter most to me, since they keep packing and airport days simpler. Before you buy, check that the 40L size fits your airline's carry-on limits and your own packing style, since a bag this size fills up quickly on longer trips.