carried a vaude 70L for australia.
make sure you have something with good hip pads to help take the load off your shoulders.
i recommend getting one of these for day tripping around town while your big packs at the hostel:
Maxpedition Rollypoly Folding Backpack
Maxpedition Rollypoly Extreme Folding Backpack
Maxpedition MERLIN Folding Backpack
definitely a nice addition to have on your travels, especially for carrying the day to day stuff for checkin out towns.
if you wanna stick with arcteryx they make their own version of a foldable day pack:
Cierzo 18 / Packs / Single Day / Under 30L
if you plan on tossing your shit in checked baggage one of these is handy too:
MEC Skyway Airline Tote - Mountain Equipment Co-op. Free Shipping Available
i used one for australia and it was great, doubled as a laundry bag when i needed it.