A nice gentle fluffy snowfall from the illusion of clouds on the ceiling. A standard cleanup spell so anything that melts is recycled into more snowflakes. A few real snowdrifts and more illusion around the edges. Drifts and a heavier ground cover outside for snowball fights and maybe some sledding. Probably a warming charm here and there so Santa's helpers and guests don't get cold feet or fingers. That about what you had in mind? Not quite off the rack spellwork, but doable.
One caveat; I can do some shielding, but technology disrupts magic and vice versa. Need to set a schedule so I can be there for the entire event, keep all the spells up and running. Otherwise, no worries.