Wife and I just came back. No need to go into details about the attractions as you can read about them elsewhere, but I do have some practical tips that I haven't read about it anywhere else before the trip.
Cellular coverage when in Iceland
Unless you have T-Mobile which provides free international coverage, you will want to buy an eSim card. Don't use Verizon or ATT's International pass -- at $10 a day, it's a rip off. I bought an eSIM from Airalo for $8 for 3 GB with a 7-day validity. I only used 2 GB of it. It would have cost $70 if you use the phone company's international pass. The key is to turn off apps that you don't need to use when you are out and about. If you don't disable their cellular data access, they will refresh automatically in the background and use up a lot of data. I only allowed cellular data usage for Google Maps and WhatsApp and AllTrails and Google Translate. Everything else is set to refresh on WiFi only. Wait until you are back to your hotel/Airbnb where you have free WiFi to upload your photos to the cloud or to access/update social media. I even found a discount code MINFEI4587 that gave me $3 off.. The $8 eSIM now only cost $5. I used the code twice on 2 different phones, so it should be good for multiple use.Gas price
They have 4 major chains. The cheapest is ORKAN, with distinct pink flags. 200 ISK per liter is a good price. A different station across the street could be 220 or 230 ISK, so it pays to check the price before pulling into the station.Bathroom stop before Golden Circle or South Coast
Selfoss is a town southeast of Reykavik about 1.5 hrs away. It's also about 1 hr away from attractions in either Golden Circle or South Coast. Everyday, we drive to Olis (a gas station with Green color scheme) and use their bathroom before continuing the trip. There are 3 other gas stations nearby, but the Olis station has the best bathroom setup - separate men/women/handicap. Orkan and Atlantasolia (blue color scheme) are self serve without bathrooms. N1 (red color scheme) is 2 blocks in town and has only 1 single unisex bathroom that often has a line of 7 or 8 ppl.Paying for parking at various attractions
They must have some kind of camera at the entrance of each parking lot. When I went to the self-pay parking meter, I would enter the license plate number and the screen would show the exact time (down to the second) when I entered the parking lot, so don't try to skip out on paying for parking. However, you don't need to use the parking machine nearest your car. People tend to wait in line at the first machine they see, but often, there are several machines later (in the visitor center or further away from the parking spots.)Water refill station at KEF:
When flying out, after security, a lot of people want to fill their reusable water bottles. There is one refill station right after Dutyfree, but the stream is very weak and slow. When we were there at 9 AM, there were close to 20 people waiting to fill their water bottles and each person takes 2 or 3 minutes, so you can imagine how frustrating that is. You can walk about 100 ft to the nearest bathroom, and there is a refill station too, but it will likely also have a line. What you can do is to go through Passport Control and straight to Terminal D (where all US and Canada bound flights are), and find a refill station by the bathroom there. Most people by then have already filled their water bottles, so the line is minimal.Boarding the plane:
None of the flights at D terminal is using jet-bridge. When you scan the boarding pass and walk through the door, you actually walk outside to board a bus. The bus has very few seats and waits until everybody is boarded then drives to the airplane. So earlier boarding groups actually have to stand longer in the bus. You are better off being the last one boarding, then you can get off the bus first and actually get onto the airplane first.