All seasons in one day
Iceland is magical and is striking to see how fast the landscapes changes from one stop to the other. consequently the weather changes accordingly so make sure to pack from sun-cream to rain-pants.
Not only if it rains outside, but also for visiting any of the waterfalls, rain proof jacket and pants would be more than super useful. If possible I also strongly advise on waterproof shoes.
as anywhere you go with so many changes in the weather, layers are important so make sure you can take off till T-shirt level at all times.
3. Swimming suite
Iceland offers a lot of bathing opportunities so don't leave at home your swimming suite
4. Flip Flops
for some reason so many people forget to take their flip flops along in trips, well, that's why I specifically add them to the list, pack them as you will need them
believe it or not, it's better to have it than to get burned :P ... just the small hand carry one should be enough
6. Big enough Memory Card
you will take picture, and you will actually take a lot of picture. I guess if you could you would take constant pictures that's how pretty everything is. So make sure you have at least 32GB with you to be able to take pictures of everything you like. .
7. Extra charging ports
it of course depends a lot where you stay but it happened to us in a lot of places not to have more than 2 power sockets, so maybe get a plug that has 2 USBs ports or a travel extension cords
8. Extra battery
if you are a phone addict, don't forget to pack an extra battery with you, you will most probably use it at some point
9. Sleeping Mask?
I read a lot that it is highly recommended to get an eye mask with you as there is never night, I had one but did not use it, it seems that you do have that awesome invention called curtains everywhere in Iceland :)
10. Get a fleece or something warm along, you'll need it (or buy locally)
as I was saying weather can get quite chilly and the weird part is when you car is showing outside temperature of 10 degrees and then you go out and you feel like -2 :)
I felt the best in leggings in the trip, I have a pair from H&M that did a brilliant job, so I do encourage you to pack at least one pair of leggings.