Don't skip the unique foods! Hakarl, for example, is rotten, fermented shark. The locals get a kick out of tourists trying it. (It's awful, but you have to!) You can find little samples of it at the farmers market in the center of town. I actually ended up on the news because they were filming us eating it!
The ring road you may have heard about, but definitely don't skip. Even if you don't do the whole thing and just go to Jokulsarlon. We stayed at a great B&B farm out that way called Guesthouse Nypugarder that is excellent...and there isn't much choice out that way!