There are a number of places within the play that you could take a look at some of the language and discuss the racism present or not present in the remarks. One particular scene that could be interesting is Act I scene ii, as Portia dismisses some of her suitors using all kinds of racial and ethnic complaints somewhat common to the time.
You could likely make the argument, as the summary and analysis does, that Shakespeare was just doing this for fun and not actually trying to spread or strengthen the racial stereotypes she refers to. You might also be able to make a case that he was racist, given the atmosphere of the times, it wouldn't be too far fetched to think so. But it would be an interesting scene to look closely at regardless.