If I could give only one piece of advice to someone in order for them to stay in shape, to be a well-rounded person, or to keep their mind sharp, it would be to challenge themselves to learn something new every single day.
This is how our mind and body are meant to work in my opinion. We are by nature inquisitive animals with a great power of adaptability. The more you challenge yourself to do something difficult, the easier it becomes and the better you get at it. The more you stick to doing the things that are easy, the less adaptable and active you become.
Parkour
We already do this in our parkour training (if you don’t I highly suggest starting). Part of what makes parkour so fun and challenging is that there are an infinite amount of new things to learn by pulling your inspiration from the obstacles in the world around us. But, you can still get stuck into habits of familiar movements. Training a thousand lazy vaults in a row will definitely make you better at that movement, but training a thousand different ways to do lazy vaults on a thousand different obstacles will make you a master of that movement (not to mention all the movements that come from it or rely on it). Change the angle, the speed, the foot placement, the hand placement, the height, the entrance, the exit…experiment, break the rules, have fun!
Crosstraining
And why stop at just parkour? Parkour is meant to increase your capacity for movement in your environment. There are a lot of different ways to move and movement arts dedicated to them. Try rock climbing, slacklining, capoeira, martial arts, gymnastics, yoga, etc. If it is challenging and hard, give it a try and see if you like it! Some aspect of it (however small) will most definitely find its way into your parkour practice and give you some new insight or a better way to answer a challenge.
As corny as it sounds, if you train the same type of thing all the time then you are training yourself into a box. And parkour is definitely not about thinking and moving in the box. Learn something new every day, and have fun with it 
I like what you’ve said in your latest post about learning something new by “Change[ing] the angle, the speed, the foot placement, the hand placement, the height, the entrance, the exit”. To me that really speaks to the differences in techniques and a principles. When following principles(to move fluidly through your landscape with little to no negative effect to your surroundings or your self), changing the technique(the angle, the speed, the foot placement) is of little consequence. This is what makes free-flow possible! However, if you ARE intimate with the principles and dis-obey them, you’re in for a world of pain, or no growth. If we as students and educators can focus on studying and teaching principles, I think our training will go a lot further. Of course, there’s no getting around drilling techniques into the thousands before the principles find us worthy of showing themselves. Thus is life, no?
Hi Tyson,
What a great site…. keep up the goood work, Thanks
Torben Deleuran
Denmark