Is it permissible to practice a mental concentration exercise focusing on a single idea that does not contradict Islamic law, for the purpose of relaxation, psychological serenity, strengthening willpower, and relieving stress?

It is permissible for a Muslim to engage in beneficial sports so long as they do not contradict Islamic law. However, it must be noted that the knowledge of Muslims, derived from divine revelation, is more beneficial and greater. If you desire happiness in this world and the hereafter, then you must turn to Allah and perform a prayer with full devotion, understanding the meanings of the Quran. This achieves immense benefits, as the Prophet, peace and blessings be upon him, said: "Whoever performs ablution like this ablution of mine, then prays two rak'ahs in which he does not speak to himself, his past sins will be forgiven." Therefore, prayer with devotion and contemplation of the Quran is sufficient from that.