If you still prefer to carry a backpack for the extra padding but don't want to feel bulky on your ride, the 6L pack from Evoc is a good choice. It's still functional with separate compartments for gear and holds a 2L bladder. A good ventilation system keeps you from overheating on hot days. And the padding will still save your back if you happen to wipe out.
Include 2L bladder
AIR PAD SYSTEM offers perfect back ventilation
BRACE LINK for optimal adjustment of the shoulder straps