Kristin Cavallari and Jay Cutler
01/07/2012 at 03:10 PM EST
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."
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
the day after her birthday outing. "Oh heck yes."