Gas – N1 is the largest gas station chain in Iceland. You can buy a prepaid ticket at the station for 3000, 5,000 or 10,000 Kronas.
The card is good if you don't want to pay by credit card at unmanned gas stations.
We received two tickets from Budget, the car rental company. One ticket gives you 8 cups of coffee at NI stations and the other gives you a small discount if you're paying by credit card.
Telephone – It's worthwhile buying an Icelandic SIM card to use for local calls, the internet and even for overseas calls.
We bought a prepaid SIM card for 2,000 Kronas, enough to last us the entire period we were in Iceland, for the three uses I mentioned (if I recall correctly, we had 100 minutes on the card).
We bought the SIM card from a company called Siminn in the city of Selfoss, but the company has many branches all over Iceland. 

Money - It's best to pay in Euros where possible so as to avoid changing money twice. All the hotels we stayed at accepted Euros, as did a number of shops. Changing money using our credit card was more worthwhile than changing Euros to Kronas at the banks in Iceland. If you're changing Euros to Kronas anyway, it's a good idea to compare the exchange rates offered by the different banks. Everywhere we went there were a number of different bank branches one next to each other and Landsbankinn was always the most worthwhile. 

Music for the car – We recommend bringing a few CDs for the car as in some parts of the island there is bad radio station reception. 

Food – The cost of food at the supermarket and in restaurants is reasonable. It's hard to find restaurants on the road but almost every gas station has a convenience store that sometimes have hamburgers and chips, soup and more. We thoroughly enjoyed the pizza at the Kebab chain in Reykjavik (that was also very reasonably priced). We also enjoyed the meal at the restaurant Buttin in Akureyri very much.

Lodging – If you’ve booked a room at a place with shared bathrooms and toilets, it's always worthwhile requesting the room closest to the bathroom. It's also a good idea to ask if there are any rooms with on-suite bathrooms and toilets – if there are such rooms available, you may be able to upgrade at a small cost.

Orientation on the road - Finding your way on the roads in Iceland is very simple. We arrived in Iceland without a GPS and we navigated our way with the use of a map alone. We found the hotels easily too, with the help of passers-by (if we needed help at all).