Got snow? This has got to be the perfect project for a winter afternoon, and the Igloo Builder's Guide has photos and directions.
Building an igloo is easy and fun. And the igloo is a great place to spend the night on a small 'expedition' in the mountains. Much warmer than a tent, and can be built just about anywhere. Building an igloo will take somewhere between 3 and 6 hours, depending on your previous experience and level of ambition...
Traditional igloo construction involves slicing hard-packed snow. When that's lacking, try the Eskimold kit: you pack snow into plastic molds, and wind up with snow blocks which are just the right size and shape for your igloo.
Related Links:
Build an Igloo
How To Make An Igloo
Igloo -- the Traditional Arctic Snow Dome
First Person: building an igloo