I'm guessing that it's still a little early for any September deals. Unlike the airlines, car rentals seem to go way down as the date approaches and then skyrocket when there is less than 10 days to go.
I've had good luck with the Entertainment coupons and code. I prefer to take DME and then rent from the Swan or Car Care Center. But sometimes you can get a better one-way rate between MCO and Disney World than you can with doing a round trip from the airport or renting/returning at the Car Care Center.
It's all a matter of being flexible with your locations and the agency that you're willing to use. My current rate for Sept. has not budged since the day that I made my reservation back in October of last year! I keep checking, but no one has come up with a better rate than what I already have.