Having a pleasant and stress-free time at the playground is all about being prepared. Leave a bag of these items in your car and you'll never have to leave early in a hurry ever again. In both cases I have two of these, one for the car and one for the house.
Though most playgrounds have a water fountain, they are often broken. We recommend to always BYOW (bring your own water).
If you are a Nassau County resident, it's worth it for you to get a Leisure Pass. The pass is $25 and good for 3 years. It provides you discounts to pools and beaches (among others) operated by Nassau County Department of Parks, Recreation and Museums. In general, a Leisure Pass is required of residents 13+ years of age. Once at Wantagh Pool or Cantiague Pool pays for the pass!