Give little clues leading up to the big moment. You could give each clue some meaning, such as stages throughout your relationship, or you could make it like a treasure hunt, where your sweetie goes from place to place and you propose at the end.
Bury the ring in a small metal box in your yard, along with a note for your beloved telling them what a treasure they are in your life. Include a line that you have a surprise and that they'll need to look for it outside. Have them search around for the treasure with a metal detector and when they find it, get down on one knee and propose.
If you'd rather not bury the ring, just make sure that you use a metal box and put a few metal items with it. Instead of having the ring in the box, put a letter or note that tells of your devotion and love and that they are your beloved, your true "treasure" in life. As they read the note, present them with the ring and ask them to spend their life with you.
Buy a ball of string or yarn and tie the beginning of the string on the front door knob of your house or apartment. Put a sign at the front door telling her to follow the string and at the end there will be a surprise. Trail the string all around your home, threading it through things, over/under furniture, etc. At the very end you will be waiting with the string tied to the ring.