Fort DeRussy Beach Park is a very large grassy park area with a large and excellent beach along the Waikiki strip. Your best bet for parking is either in the boat harbor lot (to the northwest) which has free and paid spots or at the paid lot across the street from the beach park (north of the Kalia Rd and Saratoga Rd intersection, enter from Kalia Rd).

On the grassy section you'll find a free military museum to visit along with all sorts of shady areas to play in, picnic tables, grills, tennis courts, volleyball courts, and playground equipment (for small kids). On the beach side you'll find a beautiful piece of Waikiki that's quite large and thus not as packed as some of the main (center) Waikiki areas can get. You'll have bathrooms and showers at your disposal and because you're right next to a few hotels you'll also have easy access (via a beach walk) to food and drinks at those properties or the onsite concession stands.