First of all, you and I seem to have different understanding of "backdoor". For me, if anyone speaks about a backdoor, I imagine something allowing the attacker to control the machine remotely - for example, a trojan, or RAT, or user account they can use to log in, or webshell etc.
What you are talking is something different - how to be able to run an arbitrary command as administrator (or NT AUTHORITY\SYSTEM).
There are multiple ways of course; the first thing popping into my mind is to create a scheduled task that would run any 5 minutes and execute a CMD script located in a folder accessible to the user (
%temp% would do just great - however, in my example I've used a path which is a bit easier to access).
schtasks /create /tn "TestBackdoor" /sc minute /mo 5 /tr "C:\temp\test.cmd" /RU System
Then the user can just create or edit this file, putting any command he likes - it will be executed when the scheduled task runs. For example,
Not a perfect backdoor and quite noisy also - but it's simple, and I thought the key question here was simplicity.
Credit for help with commands to create a scheduled task goes to: