I spent 9 days in Iceland at the end of May and start of June this year - did the full loop around the island on Route 1, very nice place. My first piece of advice is don't just stay in the southwest. Sure, the Golden Circle has quite a bit in it, but if you head out you will get to see wonderful things that the stop-over-visitors don't and experience it without the crowds. Some of the best stuff I did involved being the only people there, and you wont get that in the southwest! Getting to Myvatn is totally worth it, not only is it stunning, but there is plenty to see on the way.
Anyway, I took my K-1 and probably too many lenses (but I did much more than just Iceland, that's my excuse). At a minimum, you will want something wide:
(Myvatn - FA* 24mm; ~16mm on APS-C)
(On the northern road to Akureyri - DFA 28-105mm @ 28mm; ~18mm on APS-C)
You'd be surprised how often you are taking landscape shots at normal and tele focal lengths - the air is so clear your visibility just goes on and on and on. So you definately want something in the middle - say 30-75mm on APS-C:
(Siglufjörður - DFA 28-105mm @ 105mm; ~70mm on APS-C)
And you will want something with reach, because... well... this:
(Puffin - DA* 60-250 @ 250 and then cropped some; probably 300-400mm on APS-C)
And finally, some good ND filters and a sturdy tripod. Lots and lots of waterfalls, like this:
(One of god-knows-how-many waterfalls just by the side of the road - though I didn't actually use an ND filter for this one, I used the Composite mode to stack, from memory, 25 shots taken at 1/15; I kept switching between ND filters and composite mode depending on what I had remembered to take from the car).
Ok, so that is a huge range. But I think you can cover it off with the DA 15 Limited (magic lens, small, really good stopped down), the DA 55-300 (very portable telephoto, decent range, and good enough in good light) and a new DA 16-85 (weather resistant, which is all important in Iceland, great IQ, covers off the wide to mild-tele range really well). With all that you just have to hope for good photo weather.