A couple tips from our experience ...
* We tendered into Kona, and it was a wait to get into port and to get back.
* We did a walking tour that I gleaned from the Hawaii Revealed books (these were great travel guides!). After seeing the first Christian church, the king's temple, the royal vacation home and town, there was not much else to do in town (other than just shop and poke around).
We did have nachos and drinks at Huggos on the Beach, which is within walking distance of the pier. It was grandfathered in, and is the only restaurant right on the sand. Made for great views, and we walked away for about $25.
We tried to do a more expansive walking tour, but once you leave town, there is nothing but condos and homes for at least 2 miles.
They do have free shuttles to Walmart, Hilo Hatties, etc, that a lot of the crew and passengers used. They pick you up right at the pier.
Bottom line: After being quite busy in Oahu, Honolulu, etc, it was nice to just poke around Kona. Perhaps make it more of a lowkey stop and skip the car, etc.
Let me know what you think. By the way, Hilo was great with a rental car. Do you stop there?