Stoner 
The spas
Well & Being, on Princess Drive, is the big one, with a rooftop pool, sauna and an inhalation room. It's known for letting treatment guests hang around all day.
Joya, at the Omni in Paradise Valley, has hydrotherapy, a cold plunge and a pool looking straight at Camelback. Spa Avania, on Doubletree Ranch Road in Gainey Ranch, is the central favorite, with steam, sauna and strong reviews.
Agave, at the Westin in Kierland, and Luna Spa, on McCormick Parkway in McCormick Ranch, are the easy picks if you already live nearby. Sanctuary, on Camelback's north side, is the splurge with the view.
How day access works
These are resort spas, but you don't need a room to go. The common deal is that booking a treatment gets you into the facilities for the day. Rules, prices and pool access vary and change, especially in peak season, so call first and ask what your booking includes. Summer's generally when it's easiest to get in, and when an air-conditioned day makes the most sense.
The sesh rule
Resorts set their own rules, and most ban smoking of any kind on the property, so the pool deck and the parking lot aren't options. If you want to sesh, do it at home, well before you leave, and have someone else do the driving.
The real caution is the heat. Steam rooms, saunas and hot tubs hit harder when you're high, and it's easy to lose track of how long you've been in there. If you've had an edible, skip the sauna or keep it very short, drink water, and get out if you feel dizzy. Same goes for pools. Nobody swims alone, and nobody naps on a float.
The home version
Plenty of locals skip all this and do the backyard version: pool, shade, a mister and a long afternoon. Check where you can sesh if you've got an HOA or a lease. And if you're looking for a resort to actually stay at, that's a different question. This page is about a slow day off, not a place to book a room.
For other low-effort days, see things to do high in Scottsdale, or the seasons guide for when the heat is on.