1. You can go for a hardware wallet to manage it in offline storage while making a transaction. Trezor and Ledger are some of the branded names in this market.You can create your own seed offline and insert it in the machine.That will do fine for you.
2. You can go for a paper wallet too. That is the most secure in my opinion. But that is good as long as the paper is good. As you know paper could degrade over time, you need to be careful about its storage and how you can make it lasting. You should also make a backup by encrypting, so that it will remain as a hidden folder for any person other than you. 
3. Another safe option is  offline airgapped computer with Linux. 

I think you can write them on a piece of paper or screen shot the whole private key and store it in a different computer or an offline computer that doesn't go on the internet. You can also store them in a pendrive and then lock it In a bank deposit box.

If you have a lot of cash invested into cryptos that's some of the things you could do which is quite beneficial. Then there is a risk of losing the private keys to a place where you forget its location. I guess this has happened to those in the past that have bitcoins and thought btc had no value and one day btc suddenly becomes valuable and then they lose the private keys.

Do not expose the place where you store your private keys to anyone or else it is like handing over your safe deposit box keys. Then to make sure it's safe, install firewall, anti malwares software and many more to add on extra security.

Managing private keys offline is a very important thing to do for one who is a crypto expert. For me, I think having a delicate jotter is a good idea.

Why I call it delicate is because you will keep it delicate in such a way that it is not easily tempered with.

The jotter should be a private jotter or you can as well make it a dairy.

I write most of my private keys in a book.

Another way of managing private keys is saving it on the draft of your email address but since you asked about saving it offline. I suggest with go with the dairy or jotter option.