I remember a challenge to do journaling of at least 5 sentences, one about an outing away from home, one about your favorite scraproom tool, there's usually a photo organization challenge... in 2011 there was a week-long "Book of Me" challenge. (I was on a vacation at that time last year and I don't think I even checked into the forums that weekend to see what last year's were.)
The last couple of years there has also been a week long challenge that has been fun. Looking back though I'm having a hard time remembering exactly also though! I know last year there was one challenge for journaling and another for photos.
My very first year I remember the all day challenge was to do a page about why you scrapbook.