One way I reward myself after a challenging day is.....

Honestly, I don't think about rewarding myself. It depends on what about it was challenging. Was it challenging due to my own fault? It wouldn't be appropriate to reward myself then. If the day was challenging due to others, I might tell myself, "I did the best I could. Tomorrow is another day." I might watch YouTube videos, play video games, and talk it out with someone trustworthy.