Surprisingly, witch hazel flowers in winter, so that one is easy to tell...but, others? But, especially on a warm winter day...when snow is melting, but, it's not yet spring... you might like to take students for a walk. In preparing lessons, you might consider a walk where you use a key to identify trees. Cornell has a booklet for children 8 to 12 that adults will also find helpful: http://srb.npaci.edu/cgi-bin/nsdl.cgi?uid=/2007-05-23T18:23:11Z/D6C24D0D88B737E70B034467BBB04500/treeswinter.pdf
Winter, well, that's when a "twig key" comes in handy.
A site that can help you learn the vocabulary of "twigs" is: http://www.epa.gov/superfund/students/clas_act/winter/dressup.htm
Branching patterns and bark are also useful for identification.
If you are more interested in computers than walks outside, but, still want to work on winter trees, try: branching and computers...
More tree links can be found at: http://ddc.hampshire.edu/simforest/links/link_descriptions.html