Do you really want waterproof or just water resistant. Truly Water proof: backpack by Watershed.
water proof down to 300ft, which is extreme for the parks, lol
Spraying with the water resistant spray is a good idea.
Also, the backpacks that come with their own little "rain jacket" where it snugly covers the backpack is great. Im pretty sure you can buy them if it doesn't come with it.
I spray mine with waterproofing spray - the kind you use on a tent. And it works fantastic. I also put everything inside into ziplock bags. I've had alot of extremely rainy September trips.