A trick I discovered last year is that you don't have to use miles for the entire rental car payment. In my case, the rate quoted via the SkyMiles Marketplace was approx. 37,000 miles for one week, or 1,000 miles plus a cash payment of $280.
You sir, or ma'am, are awesome. The 1k mile trick saved me about $230 off of the lowest weekly rental price I could find anywhere after spending 3+ hours trying to find a deal, coupon codes, USAA discounts, AAA discounts, etc..
SM Marketplace pulled up the rental (in this case in Hawaii for a Jeep Wrangler) for 1k DSM + $351 cash, and the best rate (using Gold status at Hertz and everything) was for $582 for the week. I was about to settle for a "Ford Fusion or equivalent"
due to the rental being out of my budget, but can now roll in a Jeep! Thanks for sharing the info!