OP, be aware that the day of the week will affect the price. It will cost MUCH less to rent between Thursday-Sunday than it will to rent on a weekday; you may actually find that it is cheaper to reserve for multiple weekend days than one weekday. There is normally not a penalty for returning a car early, EXCEPT if the fine print of a discount rate requires that you keep it over a Saturday night. Pay attention to the fine print. Unlike the previous poster, I generally find that the best rental rates in resort destinations turn up about 6 weeks before the travel date, though it does pay to keep checking in case it goes lower. Most of the rental agencies do not charge a dropoff fee between locations within central Florida, so it shouldn't cost more to just take the car straight to the airport to return it.
If your plan is to go to one of the Pinellas County beaches on the Gulf Coast, the parking lots in Pinellas now use the ParkMobile app to collect payments. You can use a credit card at a pay station and put the receipt in your window, but it is easier to set up the app on your phone ahead of time, and just pay it that way, which lets you adjust the end time on the rental without walking back to the parking lot to extend payment. (If you go to Pinellas, be careful with your return time if you are catching a night flight; check Waze periodically to make sure that no major traffic jams are developing on I-4 between Tampa and Orlando. Traffic gets backed up at rush hour between those 2 cities nearly every day, and if there is a wreck on that highway, traffic can sometimes back up for hours.)
To be honest, we find that saving the beach day for the last day isn't the best choice when flying; not only do you run the risk of traffic delay coming back to Orlando, but you don't get that chance to go back to your hotel and clean up properly in your room before your flight (though we have sometimes solved that issue by packing shower items in a daypack and showering at the WDW resort pool before heading to the airport; you do have access to all of the public areas of the resort for the rest of your check-out day.)