My personal favorites are to turn off wifi if I'm not at home or work, turn off bluetooth if I'm not home, turn down brightness at night and shaking your phone to turn on the flashlight.
I've a fun one: The BlackBerry KEYone and KEY2 have a physical button called the Convenience Key that can be set to any app or shortcut. I set mine to a Tasker shortcut that changes what it does based on the day and time (mostly based around my daily commute, for bus prediction times and an e-reader).
My personal favorites are to turn off wifi if I'm not at home or work, turn off bluetooth if I'm not home, turn down brightness at night and shaking your phone to turn on the flashlight.