when you're busy with something, you always imagine the things you could do instead of working. but when you're free, you don't know what to do!
the best way to use your free time is to write down whatever comes to your mind immidiatly, so that you won't forget it next time when you're free! you could play video games or even physical games, you could go shopping or scream in an amusement park, you could go mountain climbing or visit your relatives, or you could sleep and listen to music all day.
just remember that your free time is yours and you can do whatever that'll make you happy!