Without plans doesn't mean can pack and go any time. I replied in context with the previous poster. You don't exactly need advance planning of itinerary to know what to pack. Say I decide to go trekking and I didnt bring my trekking shoes, I can always buy there locally.
That's if the toughest thing you're going to do is a day hike. In that case, I would probably head up in my sneakers.
I would have suffered tremendously either physically or mentally if not for advanced planning. I've done multiday hikes that start and end at a different place, so you have to carry everything with you but want to keep it to a minimal at the same time. Temperatures range from 30-5 degrees on most of my trips and some times it rains, so it's great to have jackets and spare base layers.
The last trip involved climbing and we brought harnesses, helmets, ropes, carabiners and shoes. I would neither want to rent nor buy harnesses and shoes. Ropes, carabiners and helmet's I'm ok with renting but not buying. Still gonna be irritating if the rental place isn't near the crag and if our bags didn't fit everything nicely. Turned out we were short of carabiners and I'm glad we checked so we managed to buy them online and get them shipped to sg on time.
God knows how much money I would have wasted if I didn't plan in advance and how many extra jackets and shoes I would have lying around.