I'm a travel agent and I am very happy to help!
In Ensenada, I think that you should just go with the ground transportation from the cruise lines because boarding will start at mid-day and there probably won't be a lot of time to visit around Ensenada. You might consider taking a taxi - the taxis in Mexico are relatively good, but ask at a hotel or at the airport for a recommendation of a licensed, approved, (i.e. reliable) taxi company.
In Honolulu, you won't need a car because they have fantastic public transportation systems. You will be easily able to get to the sightseeing destinations, shopping on public transportation and it will be cheaper than renting a car.
On the other islands, any of the major car rental companies will have offices, but double-check their location. You would probably need to take a taxi to the offices - taxis will be available at the port. In Maui, the largest car rental facility is at the airport, which is a long way from the port (basically on the other side of the island), but they might have a small location in Kaanapali.
Have a great day!