Maui - Submarine, shopping then luau - what to do with our shopping bags???

We are going to be on the POA and are considering renting a car in Maui to drive to Lahaina. I have found (not yet purchased) a package that includes a submarine ride and luau for much less than the ship is offering for the sub ride alone. We would have a couple of hours between the sub ride and luau to do some shopping and tour Lahaina. My concern is that I keep reading not to leave ANYTHING in your rental car so I'm at a loss as to what to do with my shopping bags when we go to the luau. I don't have time to drive them 45 minutes back to the ship. Any advice? Also, how long would you say the whole rental car process takes? From getting off the ship, onto a shuttle to the airport, rent the car and get back to the ship to pick up the rest of the family? Trying to decided if the cost savings is worth it by the time I rent a car, pay for overnight parking, taxi from airport at night and the loss of time. Would it be easier to just take a taxi or car service when needed and possibly skip the Maui luau and do the one offered by NCL in Kauai? We are a family of 5 with 3 children ages 7-12.

You might google "Tips To Avoid Rental Car Crime In Hawaii" for ideas about your car. As to renting your car, you will be on what is called "Maui time" while you are here, and that means you can't really forecast how long anything will take. Do factor in though that the freedom of having your own car to go where you please when you please is a big plus rather than relying on taxis or such, and there is nothing much to do where your ship docks.