I feel like I'm spamming offers at this point, but the earn rate on the MileagePlus Shopping portal is at 10 miles/$ today (normally 2 miles/$). Not sure when it goes back to 2. This one does NOT have to be paid for now (so non-prepaid rates are OK), but I'm not sure if a reservation that's modified in the future will still earn any miles at all.
Must click through the portal and book through the Marriott website. No promo codes allowed. The card you use is up to you, so double-dip with the one of your choice. I made another post with a special offer from cashback-earning US AmEx CCs at certain properties.
I would've posted a direct link, but by the time the post got moderated, the offer might not be valid anymore.