Focus on multiple challenging goals to fill up your spare time, so much so that to feel your time becomes constraint but blessed due to the amount you achieve.
Make a goal to read or memorize a certain amount of Quran per day and week
Read, reading is of few types, there’s a lighter leisure read and there’s a more in depth scholarly reading. you can derive more from. The degree of the benefit you derive from what you read is according to the content, the most beneficial being the Quran and the Hadith, then the subjects and sciences that will benefit you and others.
Improve your math skills, there’s always more ways you can improve. From doing to differential equation or infinitesimal calculus to simple multiplications
Write, I feel writing is always rewarding.
Also try implementing calisthenics workouts in your daily routines, so you can take better advantage of your spare time. The ease due the fact to that to fact you can do it anywhere at anytime, is significant