For example, if you were coming to visit me in Australia, you could get an Optus eSim for the days you are here: https://www.optus.com.au/prepaid/esim
Annoyingly, its not yet perfect. Vodafone for example can do prepaid, but requires the prepaid physical SIM to be activated first then transferred: a Vodafone store can do it of course, but that's a pain.