Perhaps a challenge to do 15 significant actions in the days/weeks/months leading up to the anniversary. How many "To Do" lists are there (unsourced, orphaned, unconnected, Data Doctor, date problems, 0 categories, 0 stickers, etc.)? If there are 15+ such lists, the participants could pick one profile from each of 15 lists to fix up. The tree as a whole would benefit and the participants may learn some new (to them) features of WT.