All commands have to be run as some user on a Linux system, so what you're really asking for is a way to allow anonymous users to run commands on your system.
You also have a clear wish to maintain security.
Now try to reconcile these two wishes.
No luck? That's because they're irreconcilable wishes.
Think about it. Do you really want CentOS to have a mode where someone can come along and start running commands on your box without logging in? If you think you could scope it to make this harmless, you haven't been paying attention to your CVEs. As I write this, there have been 10 local Linux kernel vulnerabilities in just the past 3 months. (You may get a different number of results clicking that link at a later time.) For the most part, all you need to exploit these is to have the ability to run commands as a normal user on a system with the affected feature enabled.
I suggest that if you have to see if someone can do a sysadmin job, that you set up a VM, give the prospect full root permissions on it, and let them loose.
If you're trying to see if they can fix things, purposely damage the VM before giving it to them, then set a snapshot so you can roll back to the damaged state for the next candidate.
If you're trying to see if they can set up a particular service, install a clean copy of the OS either with the service in its default state or not installed at all. Then as before, take a snapshot before giving the prospect access to the VM.