I think having various challenges on the Forum would be interesting; if people are interested in participating. You could have a special sub forum for them, one person who is going to take one on will start the thread with the rules and the begin and end dates, and then anyone else who wants to jump in can do so. However, challenges should probably be focused on quick wins, over a day or a week, let's say. If it requires only a monthly update I just don't think it would be that successful. Journals are better for the long term goals people undertake.
I personally would join some kind of Groceries thread. I would start one but my effort is so sporadic in this area my thread would be sort of lame :) Such as, spend less than X per person per month or something along those lines.
Or... you could start a sticky thread for each day of the 21 Day makeover, and anyone taking on that particular topic can post their progress or thoughts on that sticky thread?