United has a limited-time offer that lets you choose a special reward on your next Avis or Hertz rental in addition to the miles you would normally earn. You’ll receive 150 award miles for booking on united.com, an additional 75 award miles for each day of your rental (125 award miles per day if you’re a Premier member), and then you’ll also have the ability to choose a bonus. The bonus offers are either up to 1,500 bonus miles or up to $30 deposited into a MileagePlus linked TravelBank account to be used towards future United flights. Booking directly with United will net you the 150 bonus miles and you are still able to enter your AWD/CDP number on their site. However, if you prefer you can book directly on Avis.com using promo code MUAA031 or on Hertz.com with promo code 181016 with Hertz.
If you’re just looking for the maximum amount of miles on your next rental, consider these alternate Avis & Hertz options:
American – Now through May 31, 2013, American Airlines AAdvantage members can earn up to 3,000 AAdvantage bonus miles from Avis on rentals of 2 or more consecutive days at participating airport locations in the U.S., Canada, Europe, Middle East, Africa and Asia.
US Airways – Now through February 28, 2013, Dividend Miles Preferred members can earn 3,000 bonus miles on rentals of 3 or more days from participating Avis airport locations in the U.S. and Canada.
Additionally, The Weekly Flyer writes about a Hertz / American promotion that allows you to earn up to 6 AAdvantage miles per $ on your next Hertz rental.