United MileagePlus Visa Card holders: "Get 500 bonus award miles when you enroll in Visa Checkout with your United MileagePlus Visa Card and spend $5 or more"
If you were planning on spending a few bucks anyway this is a good way to get some bonus miles AND push back the expiration date of your miles. Note that you're not buying these miles.
Here is the list of the merchants that do Visa Checkout: https://checkout.visa.c
So your $5 Starbucks fix can do it, or lunch at Taco Bell.