Layers - we experienced all 4 seasons in one day while hiking at Geiranger - snow/sleet, high wind, rain, hot sun, comfortable sun - multiple times. Felt like we were constantly pulling on or taking off clothes/coats. Definitely take rain gear - a coat would be better than a throw away poncho since you could take it off and on. A backpack to put all the extra layers in when you aren’t wearing them.