I am going to Iceland this summer for a photography workshop with Focus On Nature. If anyone has ever been there and can make some recommendations for lenses I would be most appreciative. My existing line up is the 24-105 (not one of my favorites) 70 - 200 f4. I have a 24mm TS and a 100 macro, but probably won't take them. I am using these with the 1Ds Mark III.
Depends on what you like, and how much you're weight-limited. I also use an Eos-1Ds III, and find that about 60-70% of my landscapes are taken with a 70-200 f:2.8, and about 25% with the 24-70 f:2.8. I actually own the 24-105 f:4 as well, but use it mostly for a 'walking around lens' when I have the camera around my neck. The 24-70 is just a bit sharper, and it lacks the corner vignetting that is the achilles heel of the 24-105. I end up shooting quite a few panoramic sequences with the 24-70, and its lack of vignetting makes it easy to stitch even blue-sky images.
If it were me I'd add something wider, just for the occasional near/far composition that really needs a 16-20 mm focal length. I find the 16-35 f:2.8 II barely adequate; it sure would be nice to have something as good as Nikon's 14-24. Finally, something longer for the occasional 'extracted' composition that really needs 300-400 mm. If you're weight-limited the 70-200 with a teleconverter may be all you can afford to drag with you. I have a love-hate relationship with Canon's 100-400 L zoom; too many shots from it are soft, but just enough of them look terrific so I have to keep trying.
If you plan on shooting any macro subjects, try Canon's 500D two-element close up diopter. It's very heavy for a filter and certainly not cheap, but it's a featherweight bargain if you compare it to a macro lens. On the 70-200 it'll give you decent working distance and surprisingly good sharpness.