What operating system is it and what is/was the server's purpose?
If I had no other information and I needed to determine is a server was idle I would check to see which services were installed/running and then log some network activity specifically targeting those services. Linux and Windows both have logon auditing. Windows has the performance monitor for monitoring general network activity, and many built in counters of active sessions for various services. For a file server you can check for recently modified/accessed files.
I don't think any of the metrics you've listed are really good indicators of activity or inactivity since there are so many ways to get 'false positives', depending on what you're looking for. E.g. an antivirus program could use CPU cycles or the disk could be full of unused files.
If you want to tell me which OS you're using and what the purpose of the server is/was (if you know) I can edit my answer and provide you with more information.