We went in winter, well April but it was -2 degrees. We had an amazing time. Our kids rented skates and hockey sticks and enjoyed more than 2 hours on the ice in the shadow of the incredible mountains that surround the lake. We also hiked across the frozen lake to a frozen waterfall (it takes about an hour there and back). The views are breathtaking. There is also a sleigh ride (horse-drawn sleigh) that passes on the side of the lake to the waterfall if you can't hike that far.
The hotel is also beautiful (the Fairmont), if you'd like to warm up in the lobby or peruse their shops. We did not eat in the restaurant but it was a beautiful dining room overlooking the lake.More