Screen pinning is a good fix for temporarily loaning out your device and keeping only one app available. For something more comprehensive, you can set up a dedicated user account for a frequent guest. Open Settings, then go to Users, and tap Add user. This doesn't add much in the way of parental controls, but it does keep the new user's apps and settings (like home screen layouts) away from your own. To change accounts, drag down from the top of the screen with two fingers, then tap the avatar of the current user.