Turns out you can redeem your Discover card rewards for Walmart gift cards. So if you've accumulated $40 cashback, you get a $40 gift card + a $10 bonus gift card, both to Walmart.
The gift cards say Sam's Club Membership on them, but the fine print reveals they can be used at Walmart (retail or online). I've done this several times with no problems. Good deal if you're a Walmart shopper.