How should I prep a laptop for a return?
If you're pressed for time, and since you mentioned the Macbook Air was new and purchased last week, it probably was installed with OS X Lion which has full-disk encryption. If this assumption is true, then


Back up your data. I'd recommend using Time Machine (so you don't
have to re-setup your user id, install apps, etc).
create another "throwaway" admin id, and log in to your "throwaway"
admin id
and delete yourself from the Mac using System Preferences -> Users &amp; Groups preference pane.