Just add a key ring.
This was already in "Realms of Arcadia" from 1991.
You could buy a key ring for a few coins in every shop.
When you find a key, you could drag it to the ring and it was added (the key disappeared in the inventory)
Double click on the key ring and you could see a list of rings you have.
You could sinmply use the key then (it remained on the ring and was not added back to inventory when used).
That's a very elegant solution. I have always liked using specialised containers (scroll cases, gem pouches, and so on).
