On what Lot Type does a Spa Belong?

A rabbit hole spa or other kind of lot? If it's a rabbit hole (guessing it's not), the lot type would just be No Visitors Allowed - or maybe Visitors Allowed if there are other things to do on the property (I sometimes put a pool and hot tubs on the property). If not a rabbit hole, You could make it a gym or a pool if the appropriate kinds of things would be part of it. Maybe a hangout if not?

I have my spa as part of my pool lot, so I have it set to pool.

I've got mine set to hangout since it's on Garden Market which is an altered version of Garden House and has a few rabbit hole rugs in the basement.

I just placed the Serenity Retreat in a world and if I'm not mistaken in was just set to Visitors Allowed. If I wanted more sims to show up I'd put it as a hangout, but in that particular world I have plenty of other clubs, lounges, and hangouts so I left it as Visitors Allowed.

Gym and community pool. My gym in real life has a small spa section.

I set my venue with hot tubs,massage parlor and tanning booth place as just visitors allowed or hangout.It gets quite a bit of traffic as far as townies and the townies even give you free massages.Mine has a relics register worker that is a souveneir seller and doubles as a massuese.Can also change it up to sell food with the WA food register.WA added such good things to the game plus you can grab a fortune cookie on the way out. :p The little mood boost from a good fortune is a nice touch after a spa day.

Can upgrade with handiness it to only give out good fortunes so will always get the +10 three hour moodlet.

It's a rabbit hole. Choose any lot type you want and make sure it's not a residential lot.