Ideas for day/night systems on a hollow world with a tiny sun floating in the center:

Pulsing Sun: Each day, the sun expands to its largest at midday, then shrinks back down. Each night, the sun blossoms out into many tiny “stars”, spread most disparately at midnight. There is no moon. The day/night cycle is worldwide.

Shell Moon: The “moon” is a hemisphere that covers one side of the sun at a time. Tiny holes in the surface of the moon create pseudostars in the form of shafts of light. It rotates once per day.

Twin Moons: Two spherical moons orbit the sun on either side, creating two nights on either side of the planet at a time. The poles perpendicular to the moons' orbit line are in perpetual day. There are no stars.

Moon Cluster: A great number of small moons follow complex orbital patterns criss-crossing around the sun. Any given area still has roughly equal amounts of daylight and night, but the timings are difficult to predict at best.