It would be wise to resist the paradox of productivity: do not fill your free time with more work.
In fact, your capacity for creativity depends on lulls in your schedule. Are you making enough time for vacation? Are you making enough time for sleep? Are you making enough time to be bored? According to David Burkus, author of The Myths of Creativity: The Truth About How Innovative Companies and People Generate Great Ideas:
Boredom boosts creativity because of how people prefer to alleviate it. Boredom, they suggest, motivates people to approach new and rewarding activities. In other words, an idle mind will seek a toy. (Anyone who has taken a long car ride with a young child has surely experienced some version of this phenomenon.)