We did a week fly drive and did a return trip from Reykjavik to Hofn along the whole of the southern side. It's a big island but driving is a pleasure as once you leave the capital there's hardly anyone on the roads. If you have time the highlights would be the golden circle (easily doable as a day trip from Reykjavik), Selandjafoss and Skogafoss waterfalls and Jokulsaron (really impressive but a long way to drive). I plan to do a post with my Iceland photos over the next couple of weeks - this should wet your appetite!