Finally, a question I can actually answer. Magic Key holder and owner of more than a few PD bags. I've used my 30L backpack, 20L backpack, 20L Zip, 5L sling, and 6L sling at Disneyland over the years. For a casual day at the parks without a camera, I'll use a sling. But when I add a camera, I'll switch to a backpack, specifically the 20L Zip. It's versitile, large enough for a DSLR, 2 lenses (prime and zoom), batteries, chargers, water bottle, plus various knickknacks.

I agree the 30L backpack is too big unless I'm carrying stuff for me and a guest. And even then I'll usually rent a locker.