Either a car rental from MCO to Canaveral or shuttle bus. It really depends on the day and the time of day. Shuttles may not run from MCO to the port in the afternoon, and some of the car rental dropoffs by the port close at 4pm on weekends. I just did a dummy booking with Avis and their office for the port closes at 4pm on weekends, not sure about the others. A sub-compact one-way was ~$50.
You might be stuck with a taxi being the only option.