short-lived enthusiasm
One possibility is shiny object syndrome, sometimes called bright shiny object syndrome. If you Google it, you'll get lot's of hits, e.g., https://www.entrepreneur.com/article/288370, https://productiveclub.com/shiny-object-syndrome/. I'd describe "a person who has initiative and enthusiasm to start new activities but their fervor shortly quenches and they pick up new activities" as having shiny object syndrome.