Hobbies
Work and play are not separate for Spock. Most of his fulfillment is derived from his duties. He sees "relaxing" as sleeping and conserving one's energy, but does have a few diversions . . .
Chess. Kirk calls him the "chess master", and the two often play 23rd century 3D chess. Somehow the captain manages to win most of the games through "intuition". (Spock might let him win.) The computer, however, has no such instincts, and the Vulcan has better luck playing against it. These games have a serious application in Court Martial, and Kirk and Spock sometimes use chess metaphors and tactics when confronting hostile forces. Naturally, Spock's favorite game sharpens one's thinking skills.
Music. Spock's preferred mode of artistic expression is one that relies on both technical accuracy and improvisation. On the Vulcan lyre and piano, he can either "jam" or read notated compositions. He accompanies Uhura's song in Charlie X, and flawlessly plays an unknown Brahms waltz in Requium for Methuselah. Yet sadly, he is reduced to strumming an accompaniment when Kirk requests " Row, Row, Row Your Boat" in The Final Frontier.
Pets. From the selhat of his youth to the horta and the tribbles, Spock enjoys the company of critters, though he'll not readily admit it. Perhaps he likes having some way of expressing affection without displaying emotion to humans. And he likes animals, as he points out to McCoy, since "they do not talk too much."
Meditation. While music is a social activity, Spock also needs moments of privacy during which to meditate and hone his mental disciplines. Often he simply closes his eyes or clasps his hands together with forefingers extended. Ornate Vulcan artifacts in his otherwise spartan quarters help set the mood. We glimpse at Spock meditating in The Paradise Syndrome and The Cloudminders, and most memorably in The Undiscovered Country before Kirk joins him in reflecting on the course of their lives.
Conversation. Though not a social creature, Spock likes to discuss a variety of topics with Kirk and banter with McCoy. With these two he can share his thoughts and moods, and his surprisingly sharp wit. Spock can interact with his friends both during work and leisure time - each is satifying.