Sunblock Extra changes of clothes Snacks sand Toys Extra Baby wipes - all the little clean ups Extra hats for the kids. just in case
I have to reccomend two thing:
Evian makes and aresol spray water, great to cool the baby off and very gentle mist:
And one of these blankets:
It's by Tuffo. Here's a decription, you can get them on Amazon. "This generously sized all-weather blanket can mean the difference between picnicking at the park on a wet-grass day or keeping your child cooped up indoors. The durable construction, water-resistant backing, and light layer of padding make it perfect for tossing down on the sand, campground, or soccer sidelines. Easy to carry around with its shoulder straps and storage pockets, the blanket folds up to a compact and convenient size."