This is inside Mandalay Bay. I did the spa day pass and the desert stone massage in October. I think the full day for the pass is $35 which gives you access to the steam room, warm whirl pool and sauna. The desert stone massage was amazing! They used lemon scented warm oils and warm stones. I highly recommend this place my hour for a massage was around $160 I believe.

I have done several couples treatments there. I recommend.  

COUPLES RETREAT
2 Hours
$550

COUPLES MILK AND HONEY SUGAR SCRUB 50 Minutes
Let your skin naturally glow with this dry skin-removingtreatment with honey-infused Fijian sugar and coconut milk. When both are combined, the sugar breaks down and thenatural acids of the formula begin to remove dry skin.Finish this treatment with a body butter application tohydrate and nourish your skin.

COUPLES PLATINUM MASSAGE 50 Minutes
Consult with your therapist to create a customized blendof massage techniques designed to address your specificneeds at the time of your treatment: Swedish, deep tissueor a little of both!

 
THE RESORT COUPLE
2 Hours
$520

COUPLES AROMATHERAPY HYDRO-BATH
25 Minutes

COUPLES CHAMPAGNE AND CHOCOLATES
25 Minutes (my experience: this was part of the bath for our time so it's like a 40 min soak then a rinse in the big shower together)

COUPLES AROMATHERAPY MASSAGE 50 Minutes
Experience the level of relaxation that only a journey ofsense, smell and touch can create. Enjoy the soothing anduplifting benefits of unique essential oils applied with aSwedish massage technique.

 
The Aria also offers something I want to try:
"SPA SUITE
Treat yourself to a private Spa Suite and let our staff handle all the details of your customized experience. Between treatments, you’ll unwind in your own lounge area, complete with a hot tub. All Spa Suite services are reserved for parties of up to four people in 1-hour blocks of time with a 3-hour minimum reservation required."
http://www.aria.com/content/dam/MGM/aria/spa-and-salon/services/aria-spa-and-salon-pricing-brochure-feb-2016-update.pdf