I think your plan is doable the first day, does the sunset cruise start about 5pm? Docking at 8 am and given time to rent the car, that would be plenty of time to go to Waimea Canyon and make stops along the way and back to Eleele, the next day is doable but need to make sure you head back to Lihue in plenty of time, I would say with a couple hours to spare, traffic can be terrible in Kappa area. You would need to get an early start go as far as you want to go and work your way back ( at least that is what I would do). Keep in mind tunnels has very limited parking, being there early with good luck will get a spot. But there are other beaches in the area, or beach stops along the way that are nice too. There is overnight paid parking right in the port, at the shopping center there, but I have never used it.