I like Roka bags too, and also (after a recommendation on here) Foldi ones. They are very reasonably priced, fold up so small you can take a spare one, and are fixed to fit under a seat in an aeroplane. Both Roka and Foldi have airline-aware sizing, and many styles have slots in the back for sitting them on top of wheeled suitcases if you are taking that as well as carry on luggage.
For a hospital appointment (outpatient?) I recommend a Willesden sling back from Roka. They hold a lot more than you'd think from looking at them, and can be worn cross body or slung over a shoulder. Not quite a backpack, but very versatile and they come in a huge range of colours (and are washable/wipe clean).