During my past handful or years, training on a far more serious level for Ultra racing, the #strava app and community has been an extremely useful and motivating tool, not only documenting all of my activities, but also constantly promoting the notion of personal fitness challenges. I do participate in many of their challenges, but also create many of my own. And when I create them, they are usually paired with a stamp or graphic to demarcate the effort. The Außenalster, the recreational lake center-point of Hamburg, provides a nice, quick 7km perimeter loop. I'll run it at least 10 times this next 2 weeks, with sub 35:00 (current PR) as the challenge goal.
Check my strava profile here.