In reply to DancingOnRock:
It might be a bit heavy at 900g but the Osprey Talon 33 is a great lightweight day sack. Mine feels really light and it carries well, has waist belt pockets for snacks on the go and can be picked up for sub £80 at the mo if you shop around.
My favourite feature is the hydration sleeve is external so I can refill the bladder without delving into the bag (and unpacking half the contents).
The lid pocket is massive (which is ace) but tends to flop down unless the bag is full (less ace)
I was planning to get a friend of a friend in the trade to make a bespoke design bag for me but this ticked all the features I wanted tbh
https://www.ospreyeurope.com/shop/gb_en/talon-33-1