Golden Nugget has the best price for summer weekend spa treatments in AC $99 each, I had a massage and body wrap; my friend had a spa and facial. My treatments were good -- she really worked on a knot in my neck/shoulder. My friend's masseuse was a little odd and the table was inexplicably unheated. Her facial was fine.
The staff were helpful and friendly; the spa was clean and nicely decorated. But the waiting area was in the center of all the treatment rooms, which was a little noisy. Also there was no music the first part of our stay, which made everything seem even noisier (not relaxing). Coffee and caffeinated tea were offered, which seemed an odd choice for a spa (no decaf). The steam room, sauna and whirlpool were clean and nice but kind of small and not nearly hot enough for me (tho my friend thought they were fine). Many spas in AC are by the pool but at the Golden Nugget the spa does not give you access to the pool.
If you are on a budget this is fine, but if not you might want to try the Qua Spa at Caesar's.