I am leaving a spare Mavic at a friends for him to take to Iceland later next year to meet with me.

Wondered other than regularly topping up the battery level (controler only as I took all the batteries with me), what else needs to be done?

Store in a cool, dry place at typical room temp away from direct sunlight and anything magnetic like TV's/PC's etc. Also keep it sealed to prevent dust contamination building up in the drone. Note that the RC discharges it's battery more quickly than the flight batteries. I'd advise your friend to check the RC power level at least every two weeks and top it up to around 65% each time. The battery in the RC is also a LiPo and needs to be managed the same. DO NOT let it drop to 0% or you may need a new RC!

I'd throw some desiccant in the bag/case as well to absorb any moisture

Hi Fred, please store it in a cool, dry place, keep away from the high temperature and wet condition. Besides, we have the tutorials for the battery maintenance, you can refer to below.