Things we brought that were totally unnecessary:
Dinner clothes! We only dressed for 1 dinner (welcome.dinner) and everyone was comfy and casual and we were probably overdressed (I wore a dress and.wedges and husband wore pants and Cubavera shirt). All other dinners together we went to straight from activities and didn't have any chance to change. Save the space and weight and just wear your day stuff to the welcome.dinner too !
I brought a pair of sandals, some Uggs, a cute pair of fur lined rain boots, ballet flats... imagined cute pics touring town. Save that space! Ha! The only.shoes you need are sneakers and waterproof hiking boots. I did use a pair of flip flops because I always wear them.but certainly didn't need them.
Brought shorts for everyone and didn't wear them once. I'd bring 1 pair just in case but pants were definitely needed.
Our layers were: t-shirt, flannel shirt, fleece vest, rain jacket. Pants were either jeans or fleece lined leggings, and we threw on rain Pants as needed (and you definitely want to bring them) Every day that kept us plenty warm except for the glacier hike/iceberg lagoon day where we wore our heavy winter jackets. I brought 2 winter jackets because, again, pictures but that was total waste of space! The weather could always be colder and you may get more use out of a warm jacket than we did (some people wore theirs often anyway).
We did not bring any thermal layers nor did we need them.
Bring nice, warm tall socks. You'll want them for a few activities and they're great for keeping feet warm. I only brought 2 pairs and then a bunch of thin athletic ankle socks... ended up wearing my 2 tall socks most of the time on repeat!
If you want to get your hair wet at either lagoon you visit, bring your own shampoo and conditioner. The Blue Lagoon has some to use, the second baths in Mývatn just have a foamy multipurpose soap and no conditioner. They say the Mývatn water isn't as hard on your hair as the Blue Lagoon but the tip of my braid got wet and it felt the same to me

I read that a shampoo and conditioner without protein is the best option for cleaning hair after the baths, but a little research will tell you everything you want to know!
I'll add more if I think of anything ... That's what has come to my head sitting here heading home from Iceland!