First, if you have any Brits in there, get them with a display of fireworks on their pc on Guy Fawkes Day (05 November)
Now. Xmas - a small Xmas present from you with a Santa "flashing"???
My main suggestion is to hit a "Calendar" website, that can give you all sorts of ideas to carry this on throughout the year. I know we have different "days" over here, but I once had an Ash Wednesday stunt pulled on me over here - as soon as I booted the pc, the screen started descending ash over the log-on window.... stunned me to silence for a couple of seconds! (takes a lot...!! :D )
A couple of Pancake Tuesdays ago, there was a slide show did the rounds of pancakes being tossed, but sticking in various stupid places - the best one being the one that stuck to the brand new camcorder someone was testing out before giving it as a gift! :p
I'm sure you could think of loads of things to do - just apply the calendar, and get thinking!
Good luck - let me know what you turn up!
]:)
GG