The mum-of-two wore a striped crop top and patterned skirt as she strolled in the Los Angeles sunshine with little Mason on Thursday (20 June).
It seems Mason, 3, was allowed to pick out some toys during the day, as Kourtney was carrying a Lego play set.
Mason and his little sister Penelope became cousins for the first time on Saturday, 15 June, when Kourtney's younger sister Kim gave birth to her first child, a daughter, with boyfriend Kanye West.
So far no details about Kim's new arrival have been officially released, but it's rumoured she's named North West.
Kourtney hasn't publicly made any comment about Kim's baby, but sister Khloe tweeted earlier this week about how excited she is to have a new family member.
Khloe said: "I can not even begin 2describe the miracle that is now apart of our family. Mommy/baby are healthy &resting. We appreciate all of the love. More info will come when the time is right! Thank you all for understanding! We love you all dearly! Overwhelmed with love right now."