You could park at Ala Moana Center and take the pink line trolley into Waikiki. There are many stops along the trolley line and the fare is minimal. We found it easy to use, and also enjoyed wandering around the mall as well.
As for waterfront dining, Duke's or House Without a Key would be my picks. We also loved having a drink at the beach bar around the banyan tree at the Moana Surfrider. Beautiful atmosphere.
As for activities, I guess it depends on what you want to do. For us in Waikiki, we loved shopping and stopping for drinks/snacks along the strip. The beach is fun for people watching but its not a worthwhile beach day IMO. We also did Diamond Head which isn't too far, and I believe the zoo and the aquarium are in close proximity. We didn't do either, but our Diamond Head tour guide said the zoo was just ok but the aquarium was good. Hanauma Bay would make sense on a Waikiki day too.