I have a bell (brass bell by Crane), but those who hear it jump so high and seem so startled, and what I might hear as a happy ding they might hear as insistent. (Reminds me of how you can't tell the difference between a honk that means, "hey, I know you!" and one that means "I wish you weren't riding a bike in MY lane") So I've taken to just calling out, "Good morning!" or "Good afternoon!" or some other friendly greeting. They are still startled, but they realize I'm friendly.