Hard to answer this question as stated. I mean, we’ve gotten things down to the point where all we really need is wallet, keys, phone, Epi-Pen, and sun block (although even that isn’t truly necessary when we use the Badger stuff, since it is Zinc Oxide based, as long as we slather it on before heading into the park). Anything beyond that, these days, is generally unnecessary.
ETA: Wasn’t sure if this meant packing for the trip, or just what is needed in the parks.