RORY: You were right [about Madeline and Louise checking out two guys].
PARIS: And before it’s dark, they’ll have every picnic basket that’s in Jellystone Park.
Paris is referring to the Hanna-Barbera cartoon character Yogi Bear, who, together with his sidekick Boo-Boo Bear, spent all his time trying to steal picnic baskets from Jellystone National Park (based on the real life Yellowstone National Park). Yogi Bear made his debut in 1958 on The Huckleberry Hound Show, and got his own show, The Yogi Bear Show, in 1961. Since then he’s appeared in numerous TV shows, animated films, video games, and comic books.
Paris is likening Yogi and Boo-Boo’s obsession for stealing picnic baskets to Madeline and Louise’s persistence in going after guys.