In Lahaina, I highly recommend Kimos for lunch -- great views of the ocean from the patio. It was highly recommended in the Revealed book I had.
In Hilo, it's a breeze with the car rental -- it's at the airport 6 mins away and then you can just head out by making a left to the volcanoes park.
We also had time to zoom to the other side and see Rainbow Falls, Boiling Pots and one more up the road. All were about 5 mins apart. I think Akaka Falls was farther out, so it might be a little too much. But we did do all the above in the course of 4.5 hours, with one hour spent at Volcanoes Park.
Check out the Photo Galleries here for pics from our cruise. By the way, the Spirit is great -- been on her twice. Search my name under the Photo Galleries, and you will see ship photos, as well as detail photos of a balcony cabin.