Diaz hit Stella in a comfy but chic shopping ensemble - high-waisted shorts, a cream blazer and striped pumps - but what we really want to know is, what did she buy?!
We'll be keeping a close eye on the birthday girl and her party style tomorrow.
Let us know how you rate her latest look and see more of the star in our special edition, bumper style evolution below:
Hear about Cam's baby shower plans for her best friend Drew in the video below: