Try this:
Go into System Settings > Users & Groups.
Set "Automatically log in as" to "Off." (I assume you know your Mac account password. Don't lock yourself out!)
Click on the "i"-in-a-circle on the "Guest User" line, then enable "Allow guests to log into this computer". Here is Apple's description of the feature (from the Help screens):
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Allow guest users to log in to your Mac without entering a password.
- Guest users can’t log in remotely when remote login is turned off in Sharing settings.
- Guests can’t change user or computer settings.
- Files created by a guest are stored in a temporary folder, but that folder and its contents are deleted when the guest logs out.
- If FileVault is turned on, guest users can access Safari, but can’t access your encrypted disk or create files.
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Log out of your account, log into the Guest account, and see whether you have enough privileges to make use of the Home application from the Guest account. (I don't know if you will, but you can play with it.)
Before you leave, log out of your account on the Mac. With you logged out, and "Automatically log in" disabled, your friend will only be able to log into the Guest account. This account will not have access to any of your stuff stored on your internal drive (unless you have explicitly placed that stuff in some shared folder).
External drives (whether backup drives, or data storage drives) frequently aren't encrypted, so you may want to disconnect those and store them in a closet or locked file cabinet somewhere while you are away.