Then, in verse 55, it was said:إِنَّ أَصْحَابَ الْجَنَّةِ الْيَوْمَ فِي شُغُلٍ فَاكِهُونَ (The people of the Paradise are engaged today in (their) activities, happily enjoying [ them ]) - 36:55). After having mentioned the worries of the people of Jahannam, described here is the state of the people of Paradise on the day of Qiyamah that they will be enjoying themselves. The word: فَاكِهُونَ (fakihun) is the plural form of: فَاکِہُ (fakih). It signifies happiness of the heart as well as happiness out of one's surroundings. As for the expression: فِي شُغُلٍ (fi shughul) appearing before it, it could also mean that they will be totally unaffected by the misfortunes afflicting the people of Jahannam (as said by some commentators).
And then it is also possible that, at this place, this expression: فِي شُغُلٍ (fi shughul) has been added to remove the thought - when, in Jannah, there will be no religious duty like obligatory or necessary worship, nor the job of earning one's livelihood, would this lack of activity not leave one uptight or bored? - hence, it was said that their enjoyment itself will be their principal activity, and the question of any boredom simply does not arise.