iOS 10 app restrictions
This has been a long "want" of iPhone users. The closest thing you can do to this is:

Go to Settings > General > Restrictions

Enable restrictions. 

You can then enter a password, (can be different from your regular pass code), and you are able to restrict certain apps and features, but not everything. It has a small list of apps (all 1st party) and a slightly larger list of features / settings.

Source: Just did it on my iPhone 7.