Igloos are cool. What kid hasn't wanted to build an igloo? You can easily build an milk jug igloo in your classroom with some planning, lots of milk jugs and hot glue. This could be a fun winter craft for the kids when it's too cold to go out.
PVC playhouse, just add sheets! My dad made this for my one year old daughter as a Christmas present, and it has been up in our living room ever since! I purchased some pink and white sheets at Walmart and strung some Christmas lights around the top!