A few helpful tips that made our trip a success:
1. Get a TEP. This is a portable wifi Hotspot. It allowed us to use our phones as GPS devices. It was $10 a day, with unlimited data.
2. Make sure you have a PIN for your credit card. Most places were fine with a chip reader and a signature, but gas stations require PINs. This was an afterthought for us, but we are glad we did it.
3. Go swimming as often as possible. Icelandic pools have precise etiquette: take your shoes off before entering locker room, take a full shower without swimsuit, leave towel in locker room near shower & don't use your cell phone in locker rooms or pool. Embrace it & relax. Pools are the main social spot in Iceland and they are spectacular.
4. Cash isn't really necessary. We picked up a nominal amount of cash in the airport, but really only used it for tipping drivers and small purchases like coffee. And if you need it, ATMs are readily available.
5. Bring an eye mask. The sun didn't really set in the summer. Regulating our sleep was a real challenge. It was difficult to go to sleep without the cue of darkness. We also took Benadryl a few nights when we were really keyed up.