[miketoll]

I always carry a spare battery, to me an absolute must as if it runs out that is your shooting done for the day! Had that happen once and of course it was a perfect day for shooting and there was me wandering round with a lifeless camera and feeling proper miserable. Always carry a spare card too and have needed that too on occasion. I always upload photos to the computer using a card reader, easy and cheap
As far as Iceland is concerned it is not that cold in the summer and even if it is very cold I would rather have any spare battery keeping warm in my clothing. Battery grips are rather personal in that I would not want the extra bulk and weight. Apart from giving you more shots without changing batteries the only real advantage is a firing button for use when using portrait orientation. No big deal to me.
I am sure you will love Iceland, it is a truly beautiful country with amazing scenery.