"the place she falls asleep at night", I'm pretty sure. But could be "Ad meliora".
Edit: for curiosity sake, here are the events in order that Himawari has a physical appearance. "Room with the clocks" (last entity in reveal order), "Scientific Research" (eating lamen at the beach), "The place she falls asleep at night" (saves Akira), "Glued to the sky" (hanging out at the roof), "Three halves make a whole (sells watermelons)", "A life of prizes" (dance party), "Beyond the reach of god" (first unmissable contact), "ad meliora" (saves Akira again), "Hiding in plain sight" (waitress at Futaba date), the one with the japanese title (Sekai restoration), "Untitled" (quiz), "Rough cuts" (sells scarf), "Fairytale (the end until tomorrow)" (saves Ayane in the forest), "Flowerchild" (she's revealed as one of the narrators)
Some of these events can't be viewed in replay because Selebus is as bad in game design as he's good in writing (and yet has the gall to make fun of people having multiple saves, I wonder why asshat... could it be because to see Himawari in flowerchild I have to reload a save of the game at that specific part??) - and many of them require you to be a completionist or cheater.
Also, something interesting I hadn't noticed - but Himawari is unable/unwilling to shapeshift into Nodoka properly:
