I would like to ask your advice. We would like to do a light min-tracking in Perito Moreno Glacier next week. Do we need to bring hiking shoes or regular sneakers will do fine? Also, do we need a heavy jacket or light jacket + sweater + t-shirt will do fine? We don’t want to over-pack, since it’s
still summer in Argentina, but on another hand we don’t want to miss a lifetime
opportunity due to inadequate clothing. Also, do we need to book trip in advance or we can do it on the spot in El Calafate? Thank you all.

Reposting. We leave next week and I am curious on what to bring for the Glacier Excursions.
As my name suggest I am from Miami and I am thrown off by cold weather. Thanks!

Patagonia has its own sort of weather; a good deal cooler than central Argentina! You will need a sweater and fleece jacket plus a light rainproof (and ideally windproof) jacket for over the top. Weather is unpredictable in that area and you can every possible weather in one day.
If you are doing the shorter ice trek you will be given crampons to attach to your shoes. So your shoes need to be sturdy enough to take them. Sneakers are too light and also can get wet. Have a look at something a bit sturdier and waterproof if possible, or spray them with weatherproofing spray. Merrell does some lightweight boots.
You can usually pick up a tour easily when you arrive. One good company is:
http://www.hieloyaventura.com/
Have a great time!

I agree with all that the last person wrote, but add that you must have gloves and a hat as well as sunglasses. They will not let you do it without gloves, although there are some "loaners" available. We were there in mid November, and it rained, snowed and was sunny, all in the course of our trek. I assume December is warmer, but more likely to be windy. And the ice radiates its own cold, so a hat and/or scarf are nice. Some people wore ski gear, but it depends on how easily you get cold. I wore a fleece and waterproof (gortex) shell over it. We did get warm as we climbed, so layers are a good idea.

Even if you have to carry extra in your luggage, the mini-trekking is not to be missed.

Can't tell whether you need to book in advance. Our trek was pre-arranged through our hotel.

All great suggestions above! I would add good quality (preferably Merino) thermal underwear will make a world of difference to your comfort, keeping you warm but wicking away perspiration to keep you dry. Good thermal socks will keep feet warm. Layers are essential as you will get warm when trekking.

Thanks everyone for your tips!! We are all packed and ready to go!! I even ran to Target yesterday to get my son a pair of sturdy hiking shoes with ankle support. We also picked up some UV/polarized sunglasses as well.