For what does Ralph long when the boys first stop and rest?

If you're focusing on when the boys stop and rest for the very first time in the entire novel, that's in Chapter 2, after they've worked so hard to try to build their first signal fire. ("The boys lay, panting like dogs.") In that moment, Ralph notices that they've only succeeded in creating smoke, which isn't too helpful. He points out the need for actual flames. So you could say that Ralph longs for flames,...

If you're focusing on when the boys stop and rest for the very first time in the entire novel, that's in Chapter 2, after they've worked so hard to try to build their first signal fire. ("The boys lay, panting like dogs.") In that moment, Ralph notices that they've only succeeded in creating smoke, which isn't too helpful. He points out the need for actual flames. So you could say that Ralph longs for flames, or perhaps indirectly, for rescue from the island.

If you're looking in Chapter 3, when Ralph and Simon are resting after working hard at building the shelters for the entire day, Ralph definitely longs for cohesion among the group. He complains that most of the other boys won't actually do any work despite agreeing to do it during the meetings, and still the work needs to get done.

But perhaps a more significant answer to this question will be in Chapter 7, when the boys are stopping to rest in the heat of the day to eat their lunch together. It's here that we learn that Ralph longs for his body to be cleaner:

"Ralph planned his toilet. He would like to have a pair of scissors and cut this hair—he flung the mass back—cut this filthy hair right back to half an inch. He would like to have a bath, a proper wallow with soap. He passed his tongue experimentally over his teeth and decided that a toothbrush would come in handy too. Then there were his nails—"

Here, we learn that Ralph is longing for a haircut, a bath, and a good cleaning for his teeth and his fingernails. Because these longings are very civilized, and because cleanliness would dissolve the filth that the boys are wallowing in, this particular passage reveals Ralph's longing for humanity and hints that he will strongly reject the wild, dirty, violent behaviors of some of the other boys, especially Jack.