I have had the best luck at Mandalay Bay and Caesars.
Mandalay Bay, I believe you need to be a guest to use the spa (fee involved) but if you get a service (like a massage), the spa fee is waived. It's a huge place and you can come and go all-day. The staff seems to leave the guys alone. Lots can happen in that steamroom. On the other hand, there are many lingerers who will follow any 'fresh meat' into the steamroom so you have to be tolerant and not expect 'privacy'.
Caesars Qua spa is harder to get action and the attendants are fairly attending and the rooms have glass which can easily look in the rooms.
Both are great men's spa experiences overall for relaxing, recovering from the night before, a place to be uninhimted, and to be gawked at by guys you may not necessarily find attractive.
Also found action the the Flamingo Spa, the Luxor. Both have great setups for action but smaller than Mandalay and Caesars.