Be careful with port parking - not all options are legal. Here is what we learned when we cruised earlier this month.
Kahului (Maui): The Maui Mall shopping center is across from the pedestrian exit. You will see signs for Long's Drugs and Whole Foods. However, there are many signs saying that parking is for customers only. You can probably get away with an hour or so, to make a brief return to your cabin. But I would not park there overnight. The legal option is to park at Maui Beach Hotel. It was $5 for the night. The piece of mind is worth the extra walk.
Nawiliwili (Kauai): As you leave the ship, turn right on Waapa Rd and you'll see a few free parking spaces along the road. These fill up quickly. Your next option is to go to the Anchor Cove shopping center. Get an overnight parking pass for $20 from JJ's Broiler, at the far end of the shopping center.
This is correct as of now, but things may change in the future.