Snowy winter days are a great excuse to review concepts such as the water cycle, states of matter, and unit conversion. One interesting activity to try with your class that covers all of these topics is collecting and measuring snowfall. Start by asking your students to place a container with a known volume outside (if you can use a graduated container, even better), and then have them retrieve it after a snowfall.
In the meantime, ask your students to guess how much snow they will collect, how much water there will be when it melts, the temperature of the snow, and so on. You can also ask them to justify their answers using the scientific method. In general, the water content of snow can be calculated using a 10:1 ratio. In other words, 1 cm of snow, once melted, is equivalent to about 1 mm of water.
To take this activity a step further, you could build a snow board with your class and measure the snow that accumulates on it with a ruler. For greater accuracy, consider taking multiple measurements and calculating the average.
In addition, you can repeat this activity with various types of snow. This will allow your students to observe how the water content of powdery blowing snow, for example, compares to that of, say, heavier sticky snow.