Pre-flight stop at Pau Hana Poke might be a good idea since you'll be stopping at the Costco to gas up your rental before your departure.

As for shave ice, One Aloha in Kailua-Kona is our current favorite but we've not been in a few years; really nice folks running the place. If you're not interested in such a long drive, we've been meaning to try the Original Big Island Shave Ice in the King's Village.