From "car offers" on Southwest.com
Save up to 25% and earn 2,400 points per rental with National Car Rental®.
Offer valid on rentals of a midsize car or above for three days or more. Valid at participating National locations in the U.S. and Canada through May 17, 2015.
To take advantage of this offer, simply book your reservation through this page. Contract ID 5030924 will be automatically applied to your reservation. Be sure to provide your Rapid Rewards account number at the time of booking or rental to receive your points.
If you combine this with one of the coupons on the National website right now (free weekend day or $25 off weekend rental) you can earn back nearly the same value in points as the cost of the rental itself.
This weekend I've got a 3-day midsize reservation for $38 all-inclusive. Should be some of the easiest 2,400 RR points I've ever earned.