You actually get 29,997 Delta miles with 3 car rentals at Avis/Budget which is more than enough for a free Delta flight. You can find a free Delta flight starts at 25K skymiles. The cheapest days to rent car are on the weekend I believe.
From the site:
Now through August 1, 2008, you can earn 9,999 bonus miles on your next rental placed through delta.com (maximum of 3 bonuses total on both Avis and Budget).
How to Book and Qualify:
- Have your Skymiles # ready, or get one here.
- The car must be booked on Delta.com yet it does not have to be in
conjunction with a flight.
- Select one of the Avis or Budget rental options.
- Pick one day only with the cheapest car.
- Do this on three times before August 1st
- Allow 4 to 6 weeks after qualification for bonus miles to be posted to your SkyMiles account.