You can put all kinds of lights into a mason jar, but the most common are fairy lights, small LED light bulbs that resemble the original Edison light bulb, glow-in-the-dark LEDs, and twinkle lights.
And if you’re looking for something that adds a little bit more calming vibes, you can also put small candles inside your mason jar or even create mason jar oil lamps if you don’t have any candles or lights available at hand.