How can I utilize my free time in a way that pleases Allah and entertains me, while avoiding falling into sins, especially after completing my studies and not enrolling in university?

Free time is a blessing for which a servant will be questioned on the Day of Resurrection. It is out of Allah's mercy that He did not restrict His servants regarding acts of obedience; rather, their types have multiplied, and a Muslim can move between them or transform permissible acts into acts of devotion, as An-Nawawi mentioned in Riyad as-Salihin, "Chapter on the Abundance of Ways to Goodness." It is advisable to be keen on seeking knowledge, inviting to Allah, and cooperating in righteousness and piety using the available means.