updated 01/07/2012 AT 3:10 PM ET
•originally published 01/07/2012 AT 3:35 PM ET
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.”