Iceland is known for unforeseeable weather. It is likely that you can see all 4 seasons in one day.
Dressing for that kind sort of weather can always be demanding so we always suggest dressing in layers. 3 or 4 upper layers are highly recommended, for example, a light t-shirt (preferably wool) next to the skin, then a fleece or heavy wool garment topped off with a waterproof windbreaker.
Jeans are not recommended for our trips as they are not suitable for the weather conditions, they become very heavy, cold and uncomfortable to wear when they get wet.
Lightweight hiking pants, track pants or long cotton pants are convenient and wear wind/waterproof pants over. There is high UV and sunlight reflecting off on the glacier so we highly recommend using sunglasses and sunscreen even if it is cloudy.
You will need sturdy hiking shoes that fit crampons for the glacier hiking tours. If you do not own a pair then we rent out suitable hiking boots as well as a waterproof raincoat and trousers.