I once bought a ring for my girlfriend for Easter. I hid it in an egg, but it was quite a fiddly job. I'm no chef, so I bought an egg from the store and (very slowly) used a heated knife to part the seam which holds both halves of the egg together. Then I put the ring inside and kept reheating the knife and pressing it gently against the seam in order to make it whole again. Another good way of surprising her with a ring (and less time consuming) is to freeze one in a cube of ice and put it in her drink. Don't make it so subtle that she'll choke on it though.