To be authentic enough, people were asked to come with skies, sledge or other winter sport equipment and bring some small gift. Organizer brought used spruce tree - well, it was completely dry and brown, as it was tree used during last Christmas, and also christmas sweets. Illusion was nearly perfect, when we were sitting at the tree and fire, only snow and sparklers were missing.
Here you ever tried something like this?
Event link at Geochaching site