The Switchblade artifact is Ellie's personal knife, sitting in her backpack automatically at the start of Lakeside Resort. It appears in the Artifacts menu as one of the belongings you can look over while playing as Ellie.
Ellie has carried the knife since Boston; in Left Behind she pulls it on Riley when her friend sneaks up on her. In Lakeside Resort it is one of the few things she owns.
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.