I recently held a Princess birthday party for my daughters 5th birthday. Because she is a winter baby I decided to try something using the elements.  My husband spent some time with the snowblower making a good sized hill for the kids to sled on and we started freezing water in 2 litre milk cartons and 1 litre juice cartons about 1 month in advance. We also had our daughter colour them with food colouring. When the the water is frozen we store them outside in our yard and make more. I also picked up a few small pylons from the nearest dollarama and filled those with water and froze them for spires.  I also froze about 50 cups (500 ml milk woul dwork as well) for the tops of the walls.  we managed to freeze approx. 80 2 litre milk cartons, 50 1 litre juice cartons 4 pylons and 50 cups of water. The girls had a blast building the ice castle (as did us Mothers).  It stands about 3 feet high, has 4 walls, 2 entrances, 4 spires, and can fit 3 kids comfortably.  It took about 45 minutes to build and it is absolutely beautiful.  Not only was it a lot of fun for the girls but now my daughter has her very own castle to play in for the rest of the winter.  I believe we will just keep on freezing water in milk cartons and add on to the castle for as long as we can...I would post a picture of the castle but don't think I can.