Ok, I know that not every app needs trophies, but I love them, so… 
This is something that could additionally motivate as it works (usually) with activity apps.
examples:
- “You’ve earned a bronze trophy in [habit name]” // complete habit 100% times during a week
- “You’ve earned a silver trophy in [habit name]” // complete habit 100% during a month
- push notification: “Complete [habit name] once more this week and win [something magical
]”
or even just a small push:
- push notification: “complete [habit name] today to have your longest streak!”
- or: “if you complete [habit name] today, your streak will be [10/50/100]”
Because sometimes I need a little push… 