If you plan to Priceline the air tickets, plan for an extra day at each end of the trip that you are NOT counting on as play time. Since the times you get cannot be predicted, it is entirely possible that you will get a late-night flight outbound, and an early-bird flight inbound, cutting the effective length of the trip by two days. (Example: flight down leaves your home airport at 9 pm, flight home leaves Orlando at 6:30 am.) You could also get a long layover which will cause the same effect. Of course, you could get lucky and get an early flight down and a late one back, but *don't* count on that.
Pricelining hotels and rental cars doesn't affect the length of your trip; I use those all the time.