I would suggest enabling Guest access or creating a new user account. If you create a new account, make sure it has non- administrator status! Do this by going to System Preferences -> Accounts, and clicking the + button at the bottom left corner. Alternatively, you can just have them log in as a Guest, which you enable from the same pane, and after which anything they've worked on will be deleted from your machine. In both cases, you don't have to mess with your account or any of its settings, and things are just as you left them once your friend is finished with whatever they need to use your machine for.