Scott Berkun, he of inestimable software project management wisdom, offered up an idea that I think is absolutely wonderful: personal holidays. As of the time of this writing, he hasn't listed all of the ones he came up with but they were along the lines of "funny hat day" and "do something you've never done" day. Such things would certainly break up the monotony that ordinarily pervades adult life.
I'm going to have to think about possible holidays a bit more, but I think it would work even better for family (or couple) holidays where a whole family could participate. I'll update this entry with my brainstormed ideas once I do it.
[UPDATE (1/18/2006): January 18th will forever be January Fools Day. Thanks, Chris!]