I double checked policy onEnterprise.com
and also called a reservation rep.
they stated that you would have two options.
Firstly, they said that they would not allow you to park the rental vehicle at the Enterprise location in the Port Canaveral location while on an open rental. From what they described, you would have 2 options:
1. You pick up the car in Aiken and bring it back to Aiken. You would have to find and pay for long term parking (I'm assuming that they would have that somewhere near the terminal). As far as the car rental, you would be charged for every day that you have it until it's brought back to Aiken (inlcuding the days while you're on the cruise).
2. You can rent a vehicle one way - pick it up in Aiken and drop it off at the Enterprise in Port Canaveral the next day and end the rental. For your return trip you would rent another vehicle in Port Canaveral to drop off in Aiken the next day. Keep in mind that for one way rentals you're usually charged significantly more (hundreds per day or a high drop fee) than a regular rental.
You can contact an Enterprise reservation agent and have them compare prices for you for each option (after you check on the availability and pricing on the long term parking so you can add that figure in).
Hope this helps!