Retention tip #1. Idle/tycoon.

You won't see me playing idlers often, but I've recently installed Hotel Empire Tycoon and found an interesting retention mechanic:

If you don't come back to the game within a certain period of time, your offline reward stops to accumulate.

A friend of mine has told me, that lots of idle games use it for quite a while. But I write it down anyway.

And the second that caught my attention is the monetization. You can buy a manager which will instantly increase this offline rewarded period.