We just spent 5 days in Iceland and loved it. I'd recommend searching for a 5 day trip plan, I think my wife found one on Pinterest that was spot on. The best part of our trip was the diamond beach and the area around Hofn. There are rustic hot springs on a farmers field that you can visit for 5 bucks and you get a nice view of the glaciers while you soak. We stopped at a bunch of the glaciers and waterfalls which are incredible. Vik is a great place if you want to see puffins. We put some miles on but with the scenery it didn't feel like much driving at all. The golden loop was good and the national park was the highlight of the trip for my wife.
We got groceries instead of eating out to keep costs down, there is a Bonus grocery store just a couple miles away from the airport that had good prices. Oh, and the Icelandic hot dogs are very good, we found the best one was at the green gas station in Hofn. Seriously, try it.