WARNING! MANY POSSIBLE ANSWERS ARE DEADLY DANGEROUS TO THE NATIVES OF YOUR PLANET AND COULD WIPE THEM OUT!
You will want to select answers that don't threaten to kill off the natives of the planet. That means a lack of drastic changes in the amount of radiation the planet receives from the sun.
The planet's axis of rotation will probably be tilted away from exactly right angles to the plane of its orbit. That will give the planet seasons like Earth has - Earth's axis is tilted 23 degrees from exactly a right angle to the plane of its orbit. Thus at different seasons the sunlight hits a spot on Earth at different angles and with different intensities, resulting in different temperatures. Even though the amount of sunlight reaching Earth is almost exactly the same year round, Earth's axial tilt causes drastic seasonal temperature changes in various parts of Earth.
In temperate and arctic regions the temperature changes with the seasons are drastic enough that plant and animals life and humans have to adapt to it in order to survive.
The severity of your planet's seasonal temperature changes will depend on its axial tilt which you have not specified.
If the planet's sun was significantly variable the temperature changes added to the seasonal temperature changes could wipe out all life.
If the planet's orbit around it's sun was significantly elliptical instead of almost circular the temperature changes added to the seasonal temperature changes could wipe out all life.
Volcanic eruptions could change the appearance of the planet's sun drastically as they often have on Earth. They have often caused widespread crop failures and famines and one tens of thousands of years ago might have killed almost every human alive at the time. Even larger and deadlier eruptions are in the geological record.
Stars suitable for having habitable planets are all main sequence stars and have a relatively small range of mass and luminosity.
I suggest that you try something less drastic than most of the suggestions.
The eyes of the natives may be more sensitive to bright light than human eyes. Maybe they are usually active at night, and don't go out in the day more than humans go out a night.
Thus they don't get good looks at the blinding sun even as much as humans do. When they go out in the day, they notice the position of the sun from the length and direction of shadows.
And the sun could "change shape", and thus the shape of its shadow, if it was a double star. The planet could orbit around both stars if it was several times as far from the two stars as the separation between them. The stars would probably orbit each other with a period of a few Earth days, while the orbit of the planet around them might equal a few Earth months to a few Earth years.
So over a period of Earth days or weeks, the stars would appear to move apart to a distance of maybe ten or twenty degrees of arc, and then get closer together until they passed each other and grew apart on the other side, and then came back together. And their shadows, which the natives could see, would do the same, pointing farther apart as the stars grew farther apart, and moving closer together as the stars got closer together.
And maybe the planet's atmosphere would periodically change, and fill with light scattering particles, dimming the apparent brightness of the suns so the natives could look directly at them and see that they were 2 balls of light and how they moved relative to each other.
Or maybe it is the opposite. Maybe the natives are active in the day, and the atmosphere is normally so overcast that they can't see the suns, merely a blob of slightly more intense light that periodically gets wider and narrower. And perhaps there is a periodic changes in the atmosphere that make it less overcast and enables them to see the two stars clearly.
Another possibility is having a star that is slightly - repeat slightly - variable and a planetary orbit that is slightly - repeat slightly - elliptical, and both the variability and the orbit have the same period.
So either 1) the star is brighter when the planet is closer to it, and thus the two causes of temperature differences will reinforce one another, or 2) the star is brighter when the planet is father from it, and thus the two causes of temperature difference should almost cancel each other out.
And if the star's period of variability and the planet's orbit period are not exactly the same, the planet's situation will gradually change from 1) to an intermediate condition and then to 2) and back again.