I don't use a bag in the parks.
I have my DSLR on a BlackRapid strap, and I select a lens that I want to use for that particular park outing. If I want to bring another lens, I just put it in a LowePro lenscase and toss that into whatever bag my wife is bringing into the park -- usually a smaller Mickey backpack.
I like having my camera at the ready, and don't want to get weighed down with all the equipment that I don't or won't use that day.
Regards,
Boris