Make sure you know what the website goals are, and what the desired behavior of the user is supposed to be. Good usability testing can tell you more than how to setup the site so the user doesn't have to think, but also how to setup the site to direct the user to what you want them to do.

You can also sneak in some loose marketing research, if you don't pull the user in too many directions at once. I often will give the user a quick pre-test questionnaire, just to have a record of their previous web experience, surfing habits, age, gender, etc.