When you keenly observe Smart people you find these qualities in them and they either do either one of this in their free time.
Autodidacticism a.k.a Self learning .
Polymath or Expert Generalist Approach to solve problems especially the ‘Knowledge Transfer’ part
Pattern Recognition- Example - Habit loop, psychology, Data visualization, statistics
Deep Work - A work where you do more things in less amount of time a.k.a efficient work, flow state, etc
Skill Development - Any kind of skills right from speaking, drawing, to working with metals like forging, welding, machining, etc
Technically there is nothing called as free time when you are into something very deep. You just move your mind from a deep mode ( focused mode) to a shallow work ( diffused mode).
The ability to switch efficiently between these 2 modes helps you solve more problems and find more solutions very efficiently.
Let me explain you with a very classic example.
Leonardo Da Vinci
For persons who don’t know him he is a perfect example for polymath. He was a inventor, painter, sculptor, architect , scientist , musician, mathematician, engineer, Writer, anatomist, geology, astronomy, botanist, historian, and cartographer.
How was he able to do so much?
The answer lies in the above points.
Autodidactism - Da vinci didn’t have a formal education. He was a Autodidact. He selected his own topics in which he was interested and mostly self taught himself. Smart people read and gather new info and learn new stuffs during their free time.
Polymath - Since he was Autodidact he had options to explore other fields and he has tendency to derive idea and concepts from different fields into a new field or existing field which is called as knowledge transfer. The Polymath approach helped him to give solutions to many fields in a very short amount of time. Applying what your learnt completes your learning process and being a polymath gives you many opportunities to do so. Smart people explore new fields during their free time.
Pattern Recognition -Since he was a Polymath and was into Art field to he was able to observe patterns. Patterns like golden ratio, triangular composition, human figures, etc. This pattern observation helped him to create great work pieces like The Monalisa, The vitruvian man, The last supper, etc. And this pattern observation enabled him to create a spectacular machines of Renaissance age. Smart people observe patterns everywhere especially during their free time.
Deep Work - So how did he do so much work in short amount of time. The answer lies in Deep work. He had his own working den. When i say Working Den it means a place or Environment where you can do your work deeply, effectively and efficiently without any distractions. Unlike the shallow work we do these days in a distracted modern world Da vinci avoided distractions. He was an introvert and he liked loneliness. The advantages of doing deep work is you learn hard concepts within short amount of time and you do more. Second thing is your work will be unique and your work cannot be automated by machines. That is what gives value to Deep work. So if you are afraid of automation and loosing your job or business better switch to a job or business which needs Deep work. Cal Newport has an entire book on Deep Work. Most smart people prepare themselves for Deep work in their free time.
Skill Development - When i say skill a valuable skill that is very hard to learn. Let me ask you a question. How much percentage of general population can do underwater welding ? How much percentage of general population can solve differential equations in their mind ? How much percentage of general population can create hyper realistic portraits ? How much percentage of population can predict a business outcome with statistics? The answer will be in single digit percentile. Now this is what i call as skill. When you possess a valuable skill your job or work cannot be automated and that is what brings value to you. Da vinci possessed unique skills that most people cannot do. Skills like tinkering, painting, etc. Smart People Hone their skills during their free time. When you are doing activity that develops your skills you will be in a diffused mode where your entire brain works to find solution for a unsolved problem. Then again when you move to deep work you will have a solution for your unsolved problem. It works and try it for yourself.
Now ask yourself why smart people are smart ? Why Elon Musk is so successful? Why did Bruce lee become so popular ?How did Dennis Ritchie developed C language ? How did Nikola Tesla and Thomas Alva Edison invented so much ? Why Warren Buffet is so rich ?Why Conor Mcgregor is a UFC champion ?
I think after reading this you will figure out the answer for yourself.