In Honolulu, the Bus is fully accessible, and an easy way to get to Waikiki from the cruise ship port. You can also go to Ala Moana park, which has a nice beach. The shopping shuttles in Honolulu are NOT accessible for the most part, although you can get one from Hilo Hattie's or Maui Diver's with 24 hours notice.
You can also get beach chairs many places on Oahu. Here is a list:
We got accessible tours (with a lot of struggle) through Princess in Hilo (Volcano National Park) and Kauai (Waimea Canyon). The shuttle into town in Hilo is wheelchair accessible ($3.00/person). In Kona and Lahaina you are right in town, and the access is pretty easy. If you want to get out of town, rent a car (cabs are expensive). If you can transfer to a car, this is the easiest way to get around for a wheelchair user. We used Alamo and got a full-sized car most places for less than $40/day. The rental car shuttles are not accessible (except Alamo in Kauai), so plan to have an AB member of your party get the car and come back to the pier to pick you up.