As I learned from both of my trips to Iceland, base layers are key. If your itinerary involves any excursion outside, like my tour of the lava caves (which are anything but lava hot), start with a breathable base in a material like wool that, unlike cotton, won’t leave you soaked and cold in the long run. Follow it with a mid-layer like a fleece pullover or, in my case, a down vest that insulates and traps body heat. Top it all off with waterproof hiking boots (worn over wool socks) and a wind-proof or wind-resistant shell. For the latter, I went with a recycled down jacket, made from used clothing and bedding, though I would recommend a longer-fitting parka for a more extended outdoor tour.