The trick, as you've likely discovered, is to find something large enough to accommodate a water battle and essentials without being too bulky or more than you need. I find a lot of too small and too large items myself....
Maybe too big...? But I like the looks of
this sling bag.
If they made it one order smaller, I'd be there!
Other ideas:
mini knapsack
Frost River's
Field Satchel might be promising, too, but it also looks big. Or rather, I don't see listed dimensions, and "iPad-sized" can get too long, fast.
Get a
Shell Bag-- you'll be ready for anything!
-- I don't have any of these yet, but can vouch for Duluth/Frost River's quailty.
I pretty much just showed you my wish list. ;-) Your mileage may vary, but I like the (non-tactical) 'field' aesthetic, especially when paired with, say, traditional knives.
Another thought:
Baggalini. The nylon(?) bags don't improve with time like good canvas and/or leather, but they are lightweight and sport a variety of useful sizes and compartments. Search for Baggalini Sling for a variety....
Leather bags are more expensive and/or heavier...? But there are some fine items out there if that's what you're looking for. I'd start by poking around Duluth Trading, Frost River, and Saddleback.
But that's just me.
[I love love love my Saddleback medium Satchel, bought before their prices skyrocketed again, but it's too heavy to carry easily/far-- more of a car bag that I take in with me as needed than something I would happily carry out and about all day.]
Good luck, and please check back in with what you're finding. I love stuff like this.
~ P.