A quick brainstorm, based on recent use of boxplot.io and conversations with Emmett @ Gin Lane:

Basic physical activity monitoring (I use Moves)
Inbox monitoring (boxplot)
Calendar monitoring - how much free time do I have on my schedule?
Space usage in an office - especially one with shared space like Undercurrent has
Number of Chrome windows open
Number of Chrome tabs open
Number of/which native desktop applications open
Number of/which native iOS applications open
Desktop screen time per unit time
iOS screen time per unit time