I'm still using an REI convertible backpack that I bought back in 1992 and, at 25 years old, still looks like new, despite lots of travel. It has stays and a hipbelt. The shoulder straps and hipbelt can be hidden inside a fabric panel. This makes the pack look like a respectable suitcase should you want to visit a "higher class" hotel and, more important, protects the shoulder straps and hipbelt from getting caught in luggage conveyor machinery. It is carryon size for all the airlines I've encountered so far.

I have no idea if REI still carries such a pack, but it's worth looking!

EDIT, LATER: Note that this is specifically a travel pack; I use a much lighter pack for backpacking (which, here in the US, is wilderness trekking). That one is too fragile to subject to airline luggage handlers (whom I've seen at work, horrors!).