What are the recommended books and activities for someone aged 18 to make the most of their free time?

A Muslim is advised to utilize their time in what benefits them, because they will be questioned about their life on the Day of Judgment. Among the best ways to utilize time is seeking Islamic knowledge, especially that which is an individual obligation (fard al-ayn) and about which a Muslim is not excused for being ignorant. It is also possible to help disseminate knowledge, guide others to beneficial websites, and convey what has been learned to others.