After spending multiple nights together to fulfil Auden's "quest" of experiencing the childhood and teenage activities she never had, Auden and Eli can no longer ignore their chemistry and finally (finally!) go in for the kiss. In the Netflix version, it happens after a "hot dog party" and food fight, when Auden, Eli, and their friends run into the ocean for a night swim (very efficiently knocking three items off of Auden's quest list). They kiss with the waves crashing around them and their friends cheering in the background — very sweet and appropriately cinematic.
In the book, Auden and Eli's first kiss happens on a crowded dance floor at the Tallyho, a local club that Eli sneaks them into. Tallyho is a running joke in both the book and the film, but we don't get to see it brought to life on-screen. (Auden's friend Leah, the biggest — and possibly only — Tallyho fan in either version, would be offended.)