The phrase "fill free time" is correct and usable in written English.
You can use it when you are talking about how to make the most out of time off from work or school. For example: "I like to use my free time to learn something new."
There's truth to the view that people can be kept tractable as long as you fill their bellies and give them violent spectacles to fill their free time.
Raine, 65 in December, will retire next year from his Oxford post but has already made preparations to fill any free time.
Not only do you need to maintain your high grades in the classroom, it's also essential to fill your free time with volunteering and various clinical placements – not to mention the arduous task of revising for the UKCAT and BMAT exams.
The stories you read about Jeffrey making 100 to 150 phone calls a day are true.... His three secretaries have headsets and try to fill any free time during the day with calls — they will keep calling people back or try to arrange times and then when they get somebody put them through to Jeffrey with a brief comment about why he's talking to them.
Not every song on the album is as serious—"Hate It Here" somewhat humorously describes a man who tries to fill his free time with chores after breaking up with his lover.
Or if venturing out on the town isn't appealing, make sure to bring a sufficient amount of books and movies to fill up free time (make sure all those streaming services are paid off and up to date).
But that's the choice that new products like The 64, currently raising funds on Indiegogo, presents to us in 2016: do you want to smash about with your mates on Rocket League, or will Super Sprint fill that free time just as excitingly?
"They help students learn to work together and fill their free time".
"It fills my free time, and you meet a lot of people.
They slaved in the master's kitchen and filled their free time with pranks.
