Use a taxi. The rental car agencies are further away from Ron Jons than the pier and in the opposite direction. Its probably less than a mile as the crow flies (straight over the water) and just a tad bit longer using a vehicle (out and around). It was around $10 even with the tip. Being that Ron Jon makes you check out so early grab a taxi and you will be in line before the terminal opens normally.
If you chose a rental car as Jimbo said...I would rent at MCO and return it to Port Canaveral. We have used Thrifty/Dollar. Even with a drop-off fee they were the cheapest. Remember the car rental shuttle will be extremely busy on a cruise day.
We have done it both with a taxi and a rental shuttle. The taxi is easiest. Use the rental option if you are doing things outside of Ron Jon like Kennedy Space Center, Wal-Mart, etc
BTW. WE LOVE RON JON RESORT!!!! Always have so much there.