and by doing for His sake all that He loves. If a person masters this, the fruits that follow are beyond description.

As for your question about the best deeds after the obligatory duties, this varies from person to person according to their abilities and what is appropriate at different times. No single comprehensive, detailed answer can therefore apply to everyone. However, there is something approaching a consensus among those knowledgeable about Allah and His commands: constant remembrance of Allah is, as a general rule, the best occupation for a servant. This is indicated by the hadith of Abū Hurayrah narrated by Muslim:

“The mufarridūn have forged ahead.”
They asked, “O Messenger of Allah, who are the mufarridūn?”
He replied, “The men and women who remember Allah often.”

Abū Dāwūd also narrated from Abū al-Dardā’, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said:

“Shall I tell you which of your deeds is best, purest in the sight of your Sovereign, and raises you highest in rank; better for you than giving gold and silver, and better than meeting your enemies and striking their necks while they strike yours?”
They said, “Yes, O Messenger of Allah.”
He replied, “Remembering Allah.”

The evidence for this from the Quran and from faith—through direct observation, transmitted reports, and reflection—is abundant. At the very least, the servant should consistently recite the remembrances transmitted from the teacher of goodness and leader of the God-fearing, peace and blessings be upon him, such as the remembrances prescribed for specific times at the beginning and end of the day