I think it would be cool to have a data field that shows cumulative distance/time over [X time period]. Sort of a motivational thing - if I see I only have say 1.5 miles left to hit 30 for the week, maybe I'll tack on a little extra. Here's how I imagine the settings:
Track: Distance, Time
Period: Weekly, Monthly, Annual
Activities to include: Run, Walk, Bike, Swim, etc....