We make a large (gallon size ziploc) bag of trail mix (cheerios, nuts, raisins and M&M's) with us. Then each day we would fill a snack size ziploc with the mix and bring it with us. This works out real well and it travels easily. Everybody in our family likes it and it made a nice snack between meals.
I read about another family where the Mom brought a box of snack stuff. Different types of snacks like those listed here. Each morning, each person in the family would decide what snack(s) they wanted to bring to the park that day.
If you have a fridge where you are, also try freezing some drink boxes. They also fit well in the packs and they stay cold much longer.