It's not the case during all winters, but sometimes the Lac de Joux freezes entirely. When this happens, it becomes (among other things) a magnificent snowkite and ice kite (ice skating) spot.
 The wind is relatively consistent and consequently the surface is immense. If the ice is bare (without snow on top), you'll need very long race skis with freshly sharpened edges. Otherwise, ice skating is also possible, but protect yourself well and use a smaller kite size.