Photographs (symbol)
The photos that Will keeps in his room when Lou first meets him symbolize his old way of life, and Will's attitude towards the photos is a reflection of his attitude towards that period more generally. When Lou meets him, the pictures are still on display, signifying his attachment to that period and unwillingness to accept that it has ended. Alicia and Rupert's announcement that they are engaged prompts Will to recognize that he cannot return to his old life, and he violently destroys the pictures, feeling angry at Alicia and at figures from his past more generally. Eventually, the pictures end up tucked away in a drawer, which represents the fact that Will maintains an attachment to his old life but tries not to dwell on it, and has turned his attention to the future. The symbolism of Will's photos gets added depth when we learn about an old photo of Lou and Patrick, in which they look happy and in love despite being in a fight. This hints that photographs in the world of this novel symbolize not only the past, but false representations of the past.
Bees (symbol)
Bees crop up time after time in this book, and they're always associated with Lou. She wears bumblebee tights, calls herself "Busy Bee" in her online chatroom, and gets a tattoo of a bee. The insects are a symbol of Lou herself—or, in any case, of the version of Lou that has reached its fullest potential. Bees can fly, so in that sense they represent Lou spreading her wings and finding freedom, just as Will encourages her to do. But they're also industrious and helpful, just like Lou when she helps Will and works to support her family. These animals, then, are the perfect symbol of Lou when she combines both of these tendencies. Though she feels torn between freedom and loyalty, the image of the bee reminds us of how she might strike a balance.
Camilla's Garden (symbol)
Camilla's garden is a complicated symbol, because it has symbolic significance not only for the reader but for Camilla herself. The garden simultaneously expresses, reinforces, and symbolizes her feelings of hope. She takes up gardening because the liveliness of the garden reminds her of the possibility for renewal and liveliness more generally, but after Will's accident cannot bring herself to work in it, since she feels so hopeless. But she begins working on it as Will recovers, and the garden becomes more beautiful as she gains hope that Lou will convince her son to live. Significantly, Lou finds the garden beautiful, but considers it a mark of Mrs. Traynor's persnickety personality rather than a labor of love. Like the garden she tends, Mrs. Traynor is difficult to read, and her good intentions tend to become obscured. Still, though Lou does not understand what the garden means to Camilla, she and Will enjoy sitting in it and have some of their first good conversations there. In other words, since the garden is a symbol of Camilla's hope, and since some promising moments take place there, we are given to understand that Mrs. Traynor's hope for her son is indirectly nurturing Will's recovery during these moments.
The Castle (symbol)
Though Will and Lou come from different parts of town and share almost no experiences or friends, the castle is familiar to both of them. This doesn't mean that it has the same connotations for them both, though. Lou has a traumatic memory of sexual assault associated with it, while Will remembers having the run of the place as a child. In this sense the symbol is flexible, standing for the limits of Lou's past and the lost limitlessness of Will's. Together, they return to the castle and have one of their most profound conversations, in which each one confesses a great fear to the other. In that way, they rewrite each of their own histories of the building and create a new, shared one. The castle, then, symbolizes Will and Lou's love in spite of their very different pasts.
Fitness (motif)
It just so happens that Patrick is gearing up to run an incredibly intense triathlon while Will's condition stagnates. In terms of the book's deepeter themes, this isn't just a coincidence. Patrick hyper-focuses on his body's condition, attempting to control everything about it, but is unwilling to engage in the emotional work necessary for a relationship. Will, in the meantime, has lost control over his body, but over the course of the book grows a great deal emotionally. Patrick's obsession with fitness allows him to control one aspect of his life while letting him avoid another. One reason that Will is so threatening to Patrick, other than the obvious fact that they are in love with the same person, is that Will is far from running triathlons but seems like an interesting and happy person all the same. In fact, Will has plenty of problems of his own, but this is revealing nonetheless: it shows that Patrick exercises obsessively because he fears the eventual decline of his own body.