With all due respect to Rick Steves, et al, I found some glaring omissions that would greatly help a tourist get along on their first visit to Iceland. These were picked up on a one-week visit, so there are probably more.  

• Tipping: Most travel guides say that there is no customary form of tipping in Iceland. In one respect, that is true. The automated payment devices used everywhere from a sidewalk handicraft stall to big stores, offers no way to input a tip. However, if you look, a lot of places like restaurants and cafes, as well as a some convenience stores, have a tip jar near the cash register. They know tipping is a tradition here in America and they appreciate our showing our gratitude in that way. So even if it’s possible to spend an entire vacation there without using actual cash, it’s a good idea to carry an assortment of change and low-denomination bills with you. (ATMs are everywhere and easily located on Google Maps.) When I check out of a hotel, I am in the habit of leaving a rather hefty tip for the housekeeper. That is hard work that is rarely well paid.
• What I should have brought: Having a compact, pocket-size, foldable eco-bag is really helpful. Bags in Iceland almost always cost money and can be pretty flimsy. Because of their native honesty, shopkeepers in Iceland don’t think twice about someone carrying a shopping bag into a store. 
• What I’m glad I brought: Over-the-counter medicines (Tylenol, etc.). Convenience stores don’t carry any and I heard one American ask and the clerk reacted with some surprise that something like that would be carried at a shop that sells food. He told her she’d have to go to a pharmacy, but he didn’t know of any nearby. I am guessing that Icelanders are very healthy people. I never noticed a hospital in Reykjavik. If you need something, Lyfja seems to be the big retail pharmacy chain there. Their their signs feature a green outline cross. 
  
  The desk clerk at my hotel warned me about how expensive the convenience stores were compared to supermarkets. Unfortunately, I was in the city center and there were none of the big stores anywhere nearby. However, I found that the markup in convenience stores was not nearly as high as in the U.S. and their sandwiches, breads, and pastries were fresh and delicious. I’d stop by a store in the evening and pick up some fruit and pastries for my breakfast. And if you like yoghurt, you will loveskyr, the thick, creamy, protein-rich Icelandic version.
• Parking in Reykjavik: In America, one just assumes that a hotel has parking, even if you must pay extra for it. From what I could see, there are only a few hotels in the city that have parking facilities, and they seem to be far outside the city center. You can get all the details about city parking at a couple of websites here and here (a very handy interactive map with zones and prices), but here’s a quick overview, including a very valuable tip I got from the hotel’s front desk.

Parking in the downtown area is very clearly marked and the cost of on-street parking varies based on location. Blue signs will let you know what location you’re in and have the hours that parking fees are charged. You can also tell when you’re in a paid parking area because parking spaces will be outlined in white on the street. P1 parking, close to the city center, is the most expensive. That goes down to P4 as you get further away. The easiest way to pay for parking is using one of the apps that you pre-link to your payment method and license plate number. Both Parka and EasyPark can do this. On-street parking has a 4-hour limit, which is enforced by the payment apps. However, if you need longer-term parking, you can pay first with one app followed by the other after 4 hours. Parking is free from the late evening to early morning. There are parking garages, as well, and they are expensive but provide a place where you can keep a car without having to worry about feeding a meter.

In this screenshot from Google Earth, you can see that the streets to the right of Ægisgata (the street running vertically) have marked parking spaces. The portion of the street to left have none, so there are no parking fees. Though you can’t see it here, paid parallel parking spaces are also clearly marked. I haven’t done the arithmetic yet, but this must have saved a whole lot of money because the days I went on tours, my cars remained in its parking space.

• Be careful of the hot water in the hotels. Where I stayed, a few seconds after you turn it on, it is scalding hot, somewhere just below boiling. At first, I thought this was an egregious energy extravagance on the part of the hotel. Then, on my first tour, I learned that heat and hot water in Iceland costs about $10 per month to cover the transmission infrastructure. Electricity costs are comparatively low. This is the benefit of all the volcanic activity on the island. At many inland locations, just below the surface are pockets of super-heated steam. These power the generators that provide virtually of Iceland’s electricity, heat (via steam radiators) and hot water. The hot water can have a very mild sulphury smell as a result because of its hellish origins. 
• Letting the cold water run for 5 or 10 seconds will flush any sulfur odor from it. Icelandic tap water is probably the best water you’ll ever drink. I’ve read that locals feel a little resentful of tourists buying bottled water and adding plastic waste to their environment. Virtually everyone I saw had some form of non-disposable water bottle with them. 
  
  Apparently, water treatment isn’t a big thing in Iceland. It comes out of the ground pure and there is little around to pollute it. 
  
  I’m not sure how common this is, but it was new to my experience. The shower in my room had two knobs, left and right. But they were not hot and cold. The left knob adjusted the temperature and the right one the intensity of the water. 
• Never having gone whale watching in Oregon, it would be hard to make a comparison, but I imagine that it could be a hit-or-miss affair. Humpbacks and Minke whales spend their summers not far off Iceland’s coast means that you are pretty much guaranteed to see them. 
  Local whale watching tours come in two basic flavors. There are the large, multi-deck boats and largish Zodiacs. The latter has the advantage of being quick. They can get you to the whales quickly. The tours include appropriate gear to keep passengers safe and reasonably dry. The larger craft may be more appropriate if you’re carrying expensive camera gear or simply don’t like the idea of going out to sea in an inflatable boat. They also have an advantage on colder days. You can retire to the onboard café to warm up with something hot to drink and a snack.

If you’ve been there and think of anything that I and the other travel guides have missed, add a comment. When I add it to this article, I’ll make sure you get credit!