Next step - find a suitable box for your gift. It can be wooden with carved patterns on it, can be plastic, you can use the box for the ring (it all depends on the size of your "key" and your taste).
So - put key in the box. There's better and put some sort of a note explaining the purpose of the key.
Write her something like: from "little key of my big heart" to "most affectionate girl in this world." or " I want to give this little key of my big heart which is infinitely loves you! It always will find a place for you!"
Lodging is also available in the form of a simple scrap of paper, scroll engraving on the tree - again all depends on the imagination.
Now tie up the box with a bright ribbon or decorate how you wish!
Go ahead - give it to her! Do not forget to wear a mysteriously seductive, intriguing smile (or other one, no less suitable for this situation).
And at the end of advice: Do not change the lock on your heart!
Key And Heart Gift Ideas from Amazon. Buy Now!
Gorgeous X-Large Iced Crystal Heart Crown Key & Lock Charm Necklace Gold Tone ~ GIFT BOXED!13mm Dangling Key & Heart Locket Large Hole Bead - Rhodium Plated