Here are my spa and cruisy park/beach experiences living in Honolulu:
~SPAS~
Island Club is the cruisiest of all the spas. Guys walk around completely naked in the mens locker. There is a sauna, steam room, hot tub, and gentlemen's lounge all within the mens locker. It is socially acceptable and normal to be completely butt ass naked, naked as the day you were born. It's a great place for nudists. It's extremely cruisy. Guys are playing all the time. It's very much like an unofficial bathhouse. Sounds perfect, right? Nothing can be this perfect, right? Right... it's $250/month plus an annual fee, plus other sign-up fees. Residents of the building get a significant discount. There are no day passes but you can request for 1 free week to test it out.
Only go to Herbal Spa if you have made arrangements to meet someone there. Otherwise, you will spend your day alone there. It is completely empty all day long. The facility itself is OK. It's old but everything works. There are lots of places to play discreetly; the issue is finding people to join you in there.
UFC Gym has a sauna in the mens locker. Most guys go in wearing swim trunks, some wear only a towel. It gets cruisy late at night after 10PM. Most guys who train here during the day are serious about fitness and aren't looking, so be careful not to mess with guys who aren't looking. The gym itself has a very straight alpha vibe, so there's plenty of eye candy but not much play except late at night as mentioned. The mens-only sauna in the locker is a very nice amenity that is hard to find these days. It's a nice gym, too. Not crowded. $60 a month. Open 24h. You can ask the front desk for a guest pass.
YMCA Nuuanu has a sauna in the mens locker. Lots of daddies. Lots of guys go naked in there but it's not very cruisy if at all. It's a very friendly, chatty atmosphere. It's almost too wholesome. It can be a nice place if you want to be naked with other guys but there's not very much playing here.
~PARKS~
The Banyan Tree
https://maps.app.goo.gl/BfYoj3mJP6dTHeZa9 and tennis courts across the street around 8PM to 10PM. You should leave the area at 10PM as that's when the Honolulu police begin to patrol this area. Kapiolani Park officially closes at 10PM every day so the police kick everyone out at that time.
The Tree is also active in the early mornings while it's still dark out, around 5AM.
This area is extremely active. Tons of tourists and locals congregate here for sex every night. Lots of group play. It's like an outdoor orgy. It has a very anonymous vibe because of how dark it is.
~BEACHES~
Polo Beach up in north shore is the most well-known unofficial nude beach. Only go here if you've made arrangements to meet people here, because it can be very hit or miss. It can be completely empty which can be a bummer if you took a day trip and drove 40 minutes out there only to find no action.
Diamond Head Lighthouse beach is another cruisy nude beach. It's conveniently located in Waikiki. Due to its close location to touristy areas, this beach tends to always be populated at all times of the day. The nude area is directly below the lighthouse. It is naturally blocked off by cliffs on either side, making the nude area very private. There are cave-like drawbacks into the cliffs where guys can go play semi-discreetly. Everyone is completely naked at this beach. It's very cruisy. It has somewhat of an outdoor bathhouse vibe but much more chill.