When you are shopping for a backpack, you'd better look for one with comfortable padded straps and a supportive waist strap, and a rain cover is usually necessary, which is to protect your backpack against moisture, for example traveling in rainly days. Compare top loading bags with panel loading bags, and check out all the pockets also.
Besides, look for the measurements of the bag and check it fits the standard carry-on luggage bag size limitation the airlines you normally fly with. Backpack sizes are always listed in liters, which will help you estimate how much stuff you can fill inside. A good size of travel backpack is a 40L carry-on backpack.