Just returned from a thoroughly enjoyable week-long vacation. I wholeheartedly endorse Iceland as a place to empty your bank account (everything is expensive) and see beautiful waterfalls, geysers, natural rock gardens, puffins, etc.
If there is ever a next time we will:
Opt for GPS in the rental car. We couldn't rely on getting a cellular signal. Fortunately, my husband had printed out maps so we didn't get lost, although we weren't always sure of that.
Bring a picture book of native plants. There were a few familiar plants (how did dandelions get to Iceland?) but many more I didn't recognize.
Bring some hair elastics. The wind turned my hair into a mass of knots. It was so strong in one place, I took off my glasses so they wouldn't be blown off my face.
Bring some disposable gloves and bags, and remove garbage left by other tourists. :(
We hoped to buy Icelandic sweaters or some other archetypical souvenirs, only to find that most were made in China. I finally found a winter hat made locally of wool from Icelandic sheep. It didn't have a traditional snowflake pattern, but that's okay because it has puffins, which have to be some of the most appealing birds ever.