One of the most well-regarded lounges in the world is the Thai First Class Lounge and Royal Orchid Spa at Bangkok airport (BKK). The service is supposed to be spectacular, and part of the appeal is that departing First Class passengers get a free hour-long massage at the spa.
There seems to be a bit of confusion about who gets access to what when, so I’ve tried to create some flowcharts to explain the access policies.
First, spa access. Summary: you have to be departing Bangkok in a premium Thai cabin to get access. The class of your departing flight determines what services you get access to, although there are some reports that passengers arriving in Thai First and departing on a 2-class plane in Thai Business get offered an hour-long massage.
Next up, first class lounge access. Summary: only people departing in Thai or Star Alliance First Class get access plus guests. Otherwise, you need to arrive in Thai First and be departing on a same-day international Thai or Star Alliance flight to get access (but no guests). If you have some other itinerary with an arrival in Thai First (e.g. next-day flight, domestic connection, just want an arrivals lounge), then YMMV (your mileage may vary).
Hope this helps! If you want the full experience (lounge access + hour-long massage), you need to depart from Bangkok in Thai Airways First Class. Luckily for you, I have a handy dandy post that lists all of the Thai Airways routes with First Class!