The director for WandaVision, Matt Shakman, said in an interview for ScreenRant that there is at least one reference to Stan Lee: a license plate.
Matt Shakman: I mean, there certainly are some that folks have noticed that are important. A certain license plate, perhaps, in an opening title recently. We have to honor Stan Lee, of course.
But we were very careful about the breadcrumbs that we left, because it is a mystery and a puzzle box. You want to make sure that all the pieces of evidence add up. And then there are fun little easter eggs on the side - many of which people have found already, if not all of them, and then analyzed. We were trying to build this story, and make sure that we were putting out all the little pieces that would eventually add up to complete the puzzle.
ScreenRant, WandaVision's Stan Lee Easter Egg Confirmed
Here, he is referring to the license plate we see on one of the cars in the opening credits to episode 7, "Breaking the Fourth Wall". The license plate reads "WANDA" with the number "122822" above it. This is referring to Stan Lee's date of birth which is 28th December 1922 or 12/28/22 (MM/dd/[yy]yy).
In the following episode, "Previously On...", we have another reference to Lee. During the flashbacks to Wanda's past we witness her drive from S.W.O.R.D. to Westview and the license plate for her car. Under the normal details the word "EXCELSIOR" is written which is clearly referencing Stan Lee's motto and sign off.
Image taken from this ScreenRant article.