Don't forget your thermals, extra gloves, thin and thick socks, head light, water bottle and balaclava. Everything else is provided. If you are taking a 16 year old, as I did, they will love it, they don't have to wash (not advisable nor are water based skin creams) nor do they have to change very often!
I would also advise taking Lithium batteries for your camera, I did and 1 set lasted the whole week.
Also, if you are not sure about cross country skiing, ask to have snow shoes instead.
Pack playing cards, DS, books etc. Mobile phone etc. reception is good, so take your charger.
We had a great time, especially as it was Christmas. There were 4 hours of daylight, and we crammed everything into that time slot. The accommodation was excellent, the newly opened cabins at the main lodge are warm and well designed ( apart from our 'curtains' that kept falling down). We had the strange experience of returning to our cabins each day to find an extra piece of furniture had been installed - we were holding out for a plasma TV, maybe the next guests will get one... The food was filling and tasty - always plentiful, we even had turkey on Christmas day, and a preset from Santa.
The Ice Hotel is cold, -5 inside, and the beds are not very comfortable, but the sleeping bags are perfect. Two words of caution, go to the loo before you retire and don't wriggle once on the bed, the reindeer skins are surprisingly slippery.
Best laugh was dog sledding, the only thing I didn't fall off.
Thought I was an old and knackered 50 year old but I managed to do everything. Please go on this trip - it is awesome!