Lots of smaller backpacks work as excellent carry-ons and many people use then as such. My wife carries an Osprey Kestrel 28 and we use it as a carry-on. It's plenty tough enough to place under an airline seat or up in the bins. Most backpacks under 60 liters, if not filled, will fall under the size requirements for carry on luggage. You can then comfortably use the pack for treks up to a week, especially if you're staying in hostels, or not carrying lots of food. Osprey and Granite Gear are my favorite packs, but there's many out there.