After going through your routine for days, weeks, and months, it can be way too easy to turn off your brain and just coast on auto-pilot. But you owe your brain more than that. You can't let it go to waste. If you are not feeling challenged, then do whatever you can to challenge yourself today. Attempt a new sport. Try out a new hobby. See if you can learn how to cook something complicated. Put some hurdles in place and see how quickly you can impress yourself.