Just went to the Aria spa a week ago (
https://www.lpsg.com/threads/vegas-spa-cruising.298641/page-48#post-8740662), and I've stayed at the Aria in the past and have used their gym for my morning workouts.
The gym is separate from the spa and can be used by hotel guests without a spa pass (unlike Mandalay Bay, where you have to pay for a spa day pass in order to use their gym), but you do have to go past the spa check-in desk to get to the gym on the second floor of the hotel. Aria's gym is pretty large and well-equipped.
To get to the spa, you have to continue down the hallway past the gym and then take an elevator up to the spa. The spa day pass is pretty expensive, $50 on Mondays-Thursdays and $100 (!) on Fridays-Sundays, and only available to hotel guests. I was able to use the spa last week because I purchased a spa treatment (which are also more expensive on the weekends). It's a nice spa, but kind of small, and because it's so expensive, it's not usually very crowded. It has four small square jacuzzis, three of them warm and one of them cold, and it has a dry redwood sauna and a pretty good-sized steam room. If you're interested in cruising, the steam room is where it mostly happens. It's pretty dark and steamy, and if you're in the back corner or in the steam room shower area, then anyone just entering the steam room can't see you, and you'll have time to compose yourself and not get caught. The spa showers are also well-suited for action, too; the doors are frosted and actually have locks on them.
Nudity is the norm at the spa, but the Aria spa is also unique in that it has a co-ed balcony pool with entrances on either end for the women's spa and the men's spa, so swim trunks are required in that pool. Because a lot of straight couples visit the spa together, you'll see a lot of guys in the men's spa with swim trunks because they plan to go out and use the co-ed pool. However, I
have heard that some of these guys turn out to be not-so-straight in the men's spa steam room when they're not around their wives and girlfriends