Lightweight.

Small enough to fit into my "cabin bag" so that I have just one personal item when I also have a carry-on.

Large enough to hold a small guidebook, map, compact umbrella, and camera.

Ideally, expandable.

The Travelon fold-over bag is my current travel bag. http://www.amazon.com/Travelon-Lugga.../dp/B007XAI5UW

It can be expanded to a larger bag by clipping the strap to the rings at the top zipper. I swapped out the strap that the fold-over bag came with for a strap from a different bag that I liked better.

I've had similar microfiber bags from Ameribag.