The KUWTK star and her NBA "bae" worked out before enjoying the festivities, sharing a mirror selfie together on Snapchat and captioning the cute moment, "after workout pose."
Khloé, 32, showed off her toned midriff in the photo, just days after fans were convinced she was expecting her first child with Tristan, 26.
As a reward for burning calories, the couple had some fun on the huge water slide before enjoying treats, including chocolate covered strawberries and gold-dusted mini ice cream cones.
Tristan adorably cheered on Khloé as she slid down the slide into a tiny pool of water in her workout clothes, saying, "Alright, come down! Let's go," in the short video clip.
Also enjoying the fun is momager Kris Jenner, who joins her daughter, who is wearing a red St. Tropez one-piece by Fox Swim, to wish her followers a happy Fourth of July on the social media platform.
Noticeably missing from the party were Khloé's sisters — who were busy with celebrations of her own. Kourtney Kardashian was spotted in St. Tropez with her much younger boy toy Younes Bendjima while Kim Kardashian was cooking up a Jamaican feast.
As for Khloé and Tristan, it looks like things are getting serious — and this star-spangled get together was just the latest sign.
"They’re talking about getting engaged by September,” an insider told Life & Style exclusively of the couple, who started dating in August 2016.
“Tristan has asked her sisters what kind of ring she’d want,” the insider added, “and he’s planning a very romantic proposal.”
In a recent episode of the family's reality show, Khloé also seemed ready to settle down with the Cleveland Cavaliers star.
"Tristan and I definitely talk about having kids," she said. "He wants to have five or six kids with me and that's lovely. We could start at one and grow from there."