One of these
from
Here (accessories, Rucksacks, Page 2) or local store. £11.99
Has abdomen strap with pocket for keys /change (very handy on bike) 2 seperate compartments that will easily fit in a laptop, Mobile pocket on left strap, thick paddidng on back and straps. It sits very low on your back so is comfortable ( i did 120 miles yesterday with Rugby boots & training gear plus 2 ring binders full of college work and didn't get a stiff back) and it comes with extra lashing for whatever you fancy strapping to it.
It got wet the other day but hasn't had a torrential yet...it seemed to hold up fine ( put paptop in a carrier bag
)