Symbolism, Imagery, Allegory
No surprise here, but Spirit-in-the-Woods is the first and most lasting symbol we have in this book. It's the place that starts it all for Jules, and one of the last places she goes to in the book as well, effectively showing us just how little she changes over the course of her life.
When Jules comes to the end of her first summer as a camper, she feels like she's being kidnapped from a world of art and culture and interesting people, and being taken away to some place just totally unsuited to her… a.k.a. her home and family, all of whom are perfectly nice. In this way, then, the camp also symbolizes Jules's lofty ideas about herself, her sense that she's somehow superior to her beginnings. Check out how badly she wants to truly belong to what she sees as the elite crew:
Julie had never actually heard of Günter Grass, but she wasn't going to let on. If anyone asked, she would insist that she too loved Günter Grass, although, she would add as protection, "I haven't read as much of him as I would like." (1.33)
She's ready to step into the role of somebody other than who she is—she's got a plan here—shirking her actual self without batting an eye. Thing is, it works, and she becomes lifelong friends with the Interestings. So, as for feeling like she's being kidnapped when she has to return home at summer's end, in her defense, as anyone who's ever been a teen knows, leaving that behind the in-crowd isn't exactly fun.
The same impulse drives Jules back to the camp when she's older and abruptly changes careers to be a camp director. Dennis even calls her out on it, accusing her of wanting to be a teenager adored at camp instead of overseeing the day-to-day running of it. Again we see Jules not feeling like her real life suits her, and instead fantasizing of the life she might have in the woods. Unfortunately for her, this second go round she's an adult who has to deal with things like grocery orders, so the glamorous days of smoking weed with the cool kids are decidedly over.
Spirit-in-the-Woods is outside normal time, partly because Manny and Edie refuse to change it even the tiniest bit, and partly because the former campers end up creating this idealized version of the camp in their heads instead of remembering what they were really like when they were young. Even Susannah gets in on it when she visits, calling the camp the "best place on earth […] nothing is as close to heaven" (1.185). And Susannah didn't even go—she just sent Jonah.
The camp is a symbol of stability, the past, and Jules's constant dissatisfaction with anything that doesn't exactly match the feelings of possibility and belonging amongst the elite that she has during that first summer. It's pretty unhealthy, actually. Spirit-in-the-Woods is essentially a place that symbolizes Jules's constant need to be a teenager at the center of attention, even as an adult.