All suggestions below are ideas. Many more are possible (just ask ChatGPT 😂)
Essentially, fun is subjective to what you feel like doing.
For introvert - Read a book. Sketch/draw/scribble. Listen to music you enjoy. Go on a solo trip. Go to gym. Try cooking based on online videos. Clean. Take photos
For extrovert - Check bookmyshow or equivalent for events. Go to a pub/non professional meet up/non professional workshop. Organize a house party. Go on a group trip (those group travels with strangers). Go to gym. Dine out. Take photos
For singles (no family nearby) - Hang out at a friend you're close to. Or maybe go out for food together. Call somebody - an old friend, an older acquaintance, a relative, your parents. Go on a date. Or just have sex (as someone suggested but don't cheat). Take photos
For ones with family - Go out for an activity all of you enjoy. Play games together. Cook together. Take photos. Clean together. House chores together. Sit and talk.
If you're still somehow feeling excluded - Watch a movie in a hall. Look up at the sky and see if you can find shapes in the clouds (if cloudy)/dance in rain (if raining)/go for a picnic (if sunny). Call someone you haven't spoken to in a while. Go hunting for second hand goods/flea markets. Sleep.
Lastly, plan your big thing to do next weekend (which can't be done impromptu)