Charlie Luciano/Startraks
Dahvi Shira
January 07, 2012 03:35 PM

On The Hills, Kristin Cavallari was often seen partying the night away at an L.A. hotspot – but when it came to turning 25 on Thursday, the reality star enjoyed a more mellow celebration.

Cavallari marked the milestone with a small group of close pals and fiancé Jay Cutler in Chicago, a source tells PEOPLE of the celebration, which included a trip to the Shedd Aquarium and dinner at a Mexican restaurant.

“They had a great time,” the source says. “Kristin really enjoyed her low-key birthday festivities.”

RELATED: Kristin Cavallari Shows Off Her Engagement Ring at Lunch

And even though it seems like Cavallari had it all, there’s one thing that only her new age could bring.

“25 means u can rent a car,” the former Dancing with the Stars contestant Tweeted the day after her birthday outing. “Oh heck yes.”

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