Is one allowed to wear a baseball hat on Shabbat?
Shalom Elyashiv, The Shulchan Aruch (301, 40) says that one is not allowed to wear a firm brim for shade on their hat on Shabbat if it protrudes more than a handbreadth (8 cm.= a little more than 3 inches), for the same reason that you can’t open an umbrella (ohel arai). On the other hand, if the brim is floppy or soft, than it would be allowed. The adult baseball hat brims which I measured were less than that amount so accordingly, it would be ok. On the other hand, in many places it might not be considered “shabbosdik” (befitting the spirit of Shabbat to wear one), so you should ask your local halachic authority if this is the case in your community. If not, than it would be allowed (just to be sure, measure your brim before Shabbat!). With Love of Israel, Rav Ari Shvat