Mobile security is important today more than ever, which is why locking your device is a great idea. What if you want to take things a step further? Well, if you're on Lollipop you can use Google's screen pinning feature which only allows users to access whatever app has been pinned, and turning this off requires the use of the PIN code you set. If you want a little more control however, something like AuLo might be a solution. It lets you set up a master code for locking the device, and then a second (or third) PIN code for your created profiles.
The idea with AuLo is that you can set up a master code for the device/screen unlock, and then create profiles with which you can lock out specific applications so other people can't use them without knowing the PIN code for that particular profile. Once you have a profile set up you can pick and choose which apps are locked when that profile is enabled, and if you want to allow access again you can either input the PIN code or simply disable the profile at any time. You can also create multiple profiles so you can have different stuff locked for different people.
Selecting apps is no challenge as all you need to do is scroll through the list and tap on the ones you want to lock out, and that's it. Once you have profiles set and enabled you can also easily add more apps to the lockout by sliding to the left on that profile and hitting the manage button. Here you can also find the button to delete profiles if you no longer have a use for them. When you create a master code with AuLo it replaces the stock lock screen you have on your device, which might be why they state the app works best with stock Android. In any case, if you need a way to lock more than just one app, AuLo should be able to do the trick and the app is fairly easy to use with a decent user interface.