Porphyrogenitus’ point about scarcity is an important one, it’s always good to keep the concept of scarcity front and center when dealing with economic issues.
Take your example of a tanning salon in a tropical environment. Such a place would not be TOTALLY useless. It would allow people to tan at night, or during monsoon season. The question isn’t “is this a productive use of resources” but rather “would there be MORE productive uses of resources than this?” The people living in a tropical climate quite likely have numerous more critical unmet needs than a tanning salon. That being said, I wouldn’t be surprised if there WERE in fact many tanning salons in tropical climates, catering exclusively to the wealthy or tourists or what have you. There are some people out there whose priorities are such and whose current level of wealth is such that for them, the best marginal use of funds would in fact be a tanning salon.