What do I need to consider, when I chose media to store a cold/offline wallet? I'd like to stash away at least part of my Bitcoins in an offline wallet which I will put in a bank deposit box.

Which media is recommended to use for that? Should I burn a CD/DVD or put it on an USB stick? Which one has the longest expiration date? Are there other alternatives?
I wouldn't recommend CD/DVD or USB sticks. CD/DVDs don't actually have that long of a lifespan. Googling around I found various different estimates and no exact number, but the consensus is that they will probably last less than 10 years. Finding an estimate for how long a usb drive will last is a little bit more hazy, but the numbers I saw for that were around aboutish 10 years as well.

There's a bigger reason why you shouldn't use CD/DVDs or a usb drive: accessibility. If you plan on locking this thing into a deposit box and forgetting about it for a while, there's a pretty decent chance that when you come back to get it you won't be able to read it. CD/DVD drives are already used less and less, and there's a pretty good chance that 10 years from now you won't be able to get your data because your computer can only read hyper-bluray disks and the USB 12.0 connector is shaped like an octagon instead of a rectangle.

I'd actually recommend printing it out. A paper wallet printed on good archival paper with archival ink can last a very, very long time. If you do this at home, it will probably cost you some money since printers that can use archival ink aren't especially cheap, but there are some online services that can do this for you too if you are willing to trust them. You could also probably find some archival paper and archival pens and actually write out your paper wallet for not too much money.

If you really, really want to go crazy, you could have your address keys machined into a piece of steel or aluminum. If you pick a material that doesn't rust easily, and store it out of the elements so it can't erode, your wallet will essentially last for ever.