The Walkman is Ellie's cassette player, an automatic artifact already in her backpack when Lakeside Resort begins. Viewing it in the Artifacts menu prompts Ellie to say she should try to fix it.
In Left Behind Ellie mentions playing a tape Riley made for her, and the two dance to it in the Boston mall. The broken player she carries in Lakeside Resort is a link to that memory.
Braun's Firefly Pendant
Braun's Firefly pendant rests on a bench inside a gazebo next to the first cabin at the resort, in The Hunt section of Lakeside Resort. It belonged to Paul D. Braun.
Ellie's backpack
Ellie's backpack is an automatic artifact Joel picks up in the back of the butcher shop in Lakeside Resort. As he walks in, he grabs it from a low shelf, and it stays in his Artifacts menu from then on.
Joel & Sarah photo
The Joel & Sarah photo is a picture of Joel with his daughter, an automatic artifact in Ellie's backpack at the start of Lakeside Resort. You can view it from the Artifacts menu while playing as Ellie.
Kristof's Firefly Pendant
Kristof's Firefly pendant is inside a blue storage container in the coal mine during The Hunt section of Lakeside Resort. It belonged to Travis Kristof.