Best luck for me of late is to pull up rates at Southwest's website, which I think gives a discount already, then modify by clicking the link at bottom to add discount codes. Pretty much have found Costco's to be as good as anything, often reducing an additional 25%.

Last weekend got $13 a day plus taxes/fees for CLE airport on a midsize.

When traveling with a spouse, Budget doesn't charge extra ($7 a day or so) for the second driver, which is also a major reason we look to Budget first.