Agree the beach near Kauai pier is AWESOME and isn't likely to be real busy.
In Honolulu there is NOT a close beach to walk to. You can however take The Bus from the pier to Ala Moana Beach Park (about 10-15 minutes) which is a lot closer to the pier than Waikiki Beach. When pau (done) with your beach time, the Ala Moana Shopping Center (largest open air shopping mall in the USA with more than 350 shops and restaurants and over 160 dining options) is just across the street, which is where you would pick up the bus anyway to return to the pier.