First you need your container. A lightweight food style box with a lid that stays firmly in place is perfect. Size of your pack is determined by size of your park bag. Depending on what you want to lug around all day, you can have a fairly large box or one much smaller.
We all like variety, so there are several categories of items you might want in your pack.
First, beverage related items. It's pretty easy to get free tap water at parks, so you can jazz it up by bringing your own little powdered drink mixes. I like coffee, but find that in parks there is often not flavored creamer, which is a necessity to me. The shelf stable little individual creamers work great, and although you wouldn't want them loose in a bag, protected in your box, all is fine.
Next is your sweet category. You can find individual cookie packs, gummies, treat size candies and the like, or be healthy and toss in some fruit. Just be careful with chocolate, and don't include it, obviously, on a hot day.
Be sure and add some protein for a quick pick me up and to put off the need a little longer for a full scale meal. Nuts, jerky, slim jims and cheese are great choices.
Pack all your goodies much like a mini suitcase and snap the lid on tightly.
Into your park bag it goes and you are prepared for a park opening to park closing kind of day!