1) A potentiometer to adjust something - could be the maximum length of random variation or the length of “on time” at power-up.
2) A switch to bypass the the timer.
3) A function switch which could do something else like change to a different set of on/off times or with a light sensor (instead of the potentiometer), change to a mode triggered by light level (as in my original design).
(Obviously, apart from the bypass switch, additional programming needed to make these features work.)
Another variation on this theme is described on the next page…