What is the "new leisure" represented in the end of Fahrenheit 451?
One of the things removed by the book ban is "slow time," or "leisure," represented by time to think, to relax, to specifically do nothing and let the mind rest. The public media system in the book is designed to bombard the mind with useless trivia and meaningless emotion every second of the day, even during sleep, so as to distract the mind from real intellectual consideration. Montag experiences this as he tries to memorize a passage from the Bible; he is distracted by an ad for toothpaste, and he can't concentrate. When he asks Faber, he gets the explanation:
"...time to think? If you're not driving a hundred miles an hour, at a clip where you can't think of anything else but the danger, then you're playing some game or sitting in some room where you can't argue with the fourwall televisor."
Later, as Montag escapes the city, he floats down the river on his back and discovers that it is possible to "slow" time, to think about actual reality at a pace conducive to creating meaning.
The river ... gave him the time at last, the leisure, to consider this month, this year, and a lifetime of years. He listened to his heart slow. His thoughts stopped rushing with his blood.
(Bradbury, Fahrenheit 451, Google Books)
This is the "new" leisure that Montag discovers, which is in fact simply the use of time not filled with distractions. Montag uses this leisure to come to his final epiphany: time will not stop, time burns everything, so it is up to him and people like him to stop burning and start creating so that the next generation will have something concrete and useful waiting.