Properties of the rewards mentioned
Cup mixed with Kafur
Light of Beauty and Joy
Garden & Silk
In the above verse, it is described as the reward; at other places in the Quran, it is stated that the Garden will be given as inheritance (7:43, 21:105, 23:11, 39:74, 43:72). That implies ownership, and consequently a much greater role and responsibility than simply being guests in the Garden.
Recliners, Shades and Fruits within Reach
Also, noteworthy is the fact that science links direct sunlight with ageing. The Gardens of Eternity hold the promise of a wonderful immortal life, free of any suffering. The absence of direct sunlight should then be an essential condition for well-being in the Gardens.
Goblets of Transparent Silver
Mixed with Zanjabil
The above verse, read together with 76:21, informs us that the drink will be pure and cleansing (see below). Quran 56:19 describes the drink as non-intoxicating.
[Al-Qur’an 76:19, translator: Pickthal] There wait on them immortal youths, whom, when thou seest, thou wouldst take for scattered pearls.
Most translators seem to favour the role of the perpetual youths to that of serving drinks to the inheritors of the Gardens. I think it completes the picture of a royal banquet.
If the Monotheist Translation’s interpretation is considered, then the following seems more plausible: Children add joy and merriment to life. A family’s joy is incomplete without children. As there is much emphasis on the family, with repeated mention of the righteous being re-united with the righteous members of their family in the Hereafter, and these verses are mentioning the joys and delights of the garden, Perhaps it’ll be children who’ll be completing the picture of a joyous and delightful ‘picnic in the garden’.