First time in Iceland - need your best tips

I'm heading to Iceland for the first time this February for a 5-night trip and I'm excited but also a bit nervous about the winter weather and short days.

I already booked Iceland golden circle tours (you can see more here) for one full day so I can see Thingvellir National Park, Geysir, and Gullfoss without rushing around. We will be based in Reykjavik the whole time and hope to catch the northern lights a few nights too.

What are your top tips for first-time visitors in winter? Anything about packing, food, or daily stuff that really helped you?