Well for the parks, the basket of my stroller holds -
Sunscreen
Hat (if needed)
Sweatshirt
Baby wipes / Diapers if needed
Bottled Waters
Capri -suns
Some snacks
I don't really bring an extra outfit for my 3 year old since our hotel is so close, I would just go back if needed. (My 3 year old is not a spiller like my older one was!) I do bring an extra outfit for my infant and have used it almost every time!!
For the beach, we also bring towels, shovels, sand toys, buckets, swimsuits/swim diapers if warm enough, sunscreen, and snacks/drinks.
When we went last September I went with a friend and her kids, no DH's. It was a hoot for us all to park and walk to the beach with a 5, 4, 4, 3, 2 and 10 month old with all that x 6!!
Andrea