You may want to look to see how they chlorinate their spa. If it is with a SWCG, it may be fine. Why not take your test kit and test the water first before changing how someone does their spa chemistry.
I have done spa maintenance at vacation rentals. Some are meticulously well maintained by the owners and others are a $15 stop on a day of 35 stops for a guy like me. $15 doesn't buy you much, and when I have 30 more stops to do not much is what you get.
I travel with my Taylor kit and test anything my family wants to go near. Few make the cut. I have called the health dept on more than one hotel. My advice is generally to avoid spas altogether, and test pools (if you know what you are doing) before use.
I assume your definition of "shock" is "kill everything", and that your concern is about bacteria/viruses/parasites in the water. The product I would use is liquid pool chlorine (10-12%). If that isn't available, then Clorox bleach without any scents, without anti-splash, without any additives at all, in the highest concentration available.
Assuming you can't travel with a test kit, buy a tube of test strips and ensure they include a measurement for stabilizer level. The CYA level is extremely important for disinfection and sanitization - the more CYA, the less effective chlorine is. The typical ranges are 0-20 (low but fine for spas that don't get much UV exposure), 20-40 (good), 60-100 (high), and 100+ (way too high). If it's above 40, you're going to need a lot more chlorine to have the same kill power as chlorine operating in a 0-20 CYA environment. In addition, the higher the pH, the more chlorine you need to use to have the same kill power.
Use the strips to ballpark the various numbers, plug those numbers into PoolMath, then add enough chlorine to hit 20ppm FC, then every few hours, test & add more chlorine to top up to 20ppm FC, and keep doing that to hold 20ppm for 24-48hrs. This will "shock" the spa and kill the vast majority of any unwanted bacteria/viruses/parasites/etc. If you have less time, aim for 10ppm FC, circulate for 1-2 hours, and then it should be usable. If you're in a rush, dose it to 6ppm, circulate for 10-20 minutes, then you can jump in right away.
If the CYA level is over 100, you can dose it to 10-12ppm and get in (after mixing) without it bleaching your skin and swimwear, since the higher the CYA, the more the chlorine is blunted. Every hour that each person spends in the spa will eat up a chunk of chlorine, and you generally don't want it to go below 3ppm, so when you're down to 2ppm, top it up to 6ppm to continue use.
I recommend against using MPS (aka non-chlorine shock) due to many people having sensitivities/allergies to it. People may confuse the allergy with a common itch-inducing bacterial infection, and the allergy can make the sensitized/blistered skin more susceptible to such infection.