Since You've Been Gone by Morgan Matson

It was Sloane who yanked Emily out of her shell and made life 100% interesting. But right before what should have been the most epic summer, Sloane just…disappears. All she leaves behind is a to-do list.
On it, thirteen Sloane-inspired tasks that Emily would normally never try. But what if they could bring her best friend back?
Apple picking at night? Okay, easy enough.
Dance until dawn? Sure. Why not?
Kiss a stranger? Um...
Emily now has this unexpected summer, and the help of Frank Porter (totally unexpected), to check things off Sloane's list. Who knows what she’ll find?
Go skinny-dipping? Wait...what?