Hertz Comiso is the nearest Hertz car hire firm. Booking a flight through RyanAir allows you to book a car from Hertz with a discount (or see booking a car for Comiso from Hertz website).
They pick you up from the airport and take you to the office to get the car, then when you return the car they bus you to the airport.
Note: Don't forget your driving licence and (from the UK) the code that is allocated online to replace the paper version of your licence. The code only lasts for a short period so if you are making a connection ensure you get the code in time.