In addition to the Buy One Get One at Wynn and Encore, there’s also a Buy One Get One going on at Mandarin Oriental.
Second night free when you book two nights, promo code is E1320531. Valid for stays through February 28.
This is actually a hotel promotion, bookable by anyone. But you can book through American Express Fine Hotels and Resorts to receive those additional amenities (room upgrade based on availability, daily breakfast for 2, $100 spa credit). You can also book via a Virtuoso agent to pick up amenities.