Linnie, I sailed the Dawn on 8/31. Disembarkment started about 9:00am and the ship left port at 10:00pm (sounds like it's now earlier). There were approximately 40 'fifty passenger coach bus' shuttles that went to the Orlando attractions. The cost was $28 per person. If you purchase tickets from NCL you can go right into the park without waiting on ticket lines. So, as you say, you won't have to park a rental (or pay for parking), and the shuttle lets you manage your time as you decide. It's the painless alternative.
The rental car option can save you big bucks. The drawback is that the rental facilities close early; However they do provide for late returns with a key 'dropoff' (several passengers did not like this option). You also have to make your way back to the ship. But if the ship now sails at 6:30pm (as mentioned above) the shuttle wouldn't buy you much time in the latter part of the day.
Port Miami rental... there are no rental facilities at the port. Although we had a confirmed reservation with Thrifty/Dollar, they never picked us up as they promised. My wife waisted almost an hour on the phone with them. Eventually we took a cab to South Beach @ $15 each way (similar to the cost of the rental, and painless). The Fontainbleu will probably be a few bucks more.
