Princess Louisa in the winter is a must do.
I was there in Febuary on 2007 and it was spectacular at that time of year. When I was there the float was covered with about a foot of snow. The sky was clear and the mountains were snow covered. Every once in a while I would see small avalanches come down from the steep rocky mountains. At the particular time I was there, the sheet of ice started just about when you come around the steep rounded cliff on the port side before you can see the float. It was a thin sheet and as I passed through, these giant sheets would slide across and out of the way. It was no problem but having someone on the bow to break up the ice with a pike pole would be helpful. The ice did take a little bottom paint off near the water line but it wasn't a big deal. As Faster mentioned, the outflows can be a challenge. You won't get any VHF reception while there, so if your there a few days, you may be in for some surprising winds once you clear Malibu Rapids. It's unlikely you will run into any boats up there except the watchman at the lodge at Malibu Rapids.
Well worth the trip in the winter, I give it 5 stars out of 5.
