Siri would be your best source of tips & tricks - she could write a book. (..? well, is it begun?)
Anyway, we traveled there together a few years ago - before the costs went astronomical. As to food, it helped to develop a taste for skyr and fruit, some biscuits, bakery bread and cheese. The now and then restaurant meal, or a stop for coffee was a treat (and left some cash for evenings at the bar). Of course it helps that the country is so intensely distracting that as long as your stomach has something in it, the "interest factor" of food can be at the end of your list. And the weather keeps things chilled so not to spoil in the car or your pack.
I get in on the 24th :-/
Oh, and these are tired old lectures, I'm sure - but watch for the wind direction when opening your car door or it may become a bonnet ornament - and those pesky (cute) sheep really do fly up out of the ditch suddenly onto the roadway. They keep their wings tucked in until ready to pounce. And now that I'm even a few years older, I am not at all sure my heart can take the thrill of those long one-way bridges anymore. I'm fairly certain that Icelanders are telepathic in this regard.