Something like this.....
Fjallraven Vintage. It comes in 20 & 30l sizes.
It's a close-weave, cotton body with leather closures. It's not water 'proof' but i've had mine out in a good rainstorm & it didn't get soaked inside. The straps are buckle adjustable, cotton padded webbing, with a padded waistbelt.
It has an internal plastic support frame & mesh back panel to keep your back cool.
I've been using one of the 30l for about 4 years & love it! It's got all the looks & style of a pre-war pack, with the comfort & carryability of a modern rucksack