Last year we did about 3 separate contests all at the same time during the month of October, I suppose we could have done them all again this year but its too late now. We do have one Halloween contest going though. We also have a Halloween themed logo and the staff have pumpkins in their rank images.
I usually have a halloween background, change our logo and stuff, but we don't do too much on my forum. (It's kinda boring, we don't celebrate all the holidays) however on past forums we used to do pumpkin carving contests, people would carve their pumpkins and post it on the forums and the winners would get free stuff from the shop or gift cards to Starbucks. It was a super fun contest, seeing all the weird pumpkins, everyone ended up getting a free item in the shop for participating in the end, because they were all too awesome!
We don't really observe holidays as consistently as we used to, but I always put up breakable Xmas lights by the end of December until January. The falling snow plugin turned out to be very incompetent with our forum software, so we abandoned that.