If you have access to them, you might check out Camelbak's hydration + cargo military line. Very well made, some good thought went into the organization of internal & external pockets, and they are somewhat "modular". Can't remember what model I ended up with, but the large, padded hipbelt and aluminum-reinforced framesheet are both removable to get a lighter weight platform when you're not packed heavy.
EDITED TO ADD: Just figured out the model I have is the original "Motherlode" made of 1000D Cordura (~2,100 cu in). I've used this pack in all the functions you're describing- carry-on, hunting, and knockabout day pack and it's done a good job of being big when I need it and small/manageable when I don't. The only complaint Camelbak got was the weight, so they redesigned it with 500D but also changed some of the organization (for the worse in my opinion). So I can't comment with experience on currently available designs, but I can vouch that their tactical line is extremely well made and a good value.