Picked up an Asian Style cowbell chime while vacationing in Lake Placid.
As soon as I saw it, I knew it would work well in the Zen garden, especially because of the sound. It has a nice deep tone, sort of like a gong, but not the kind you slam with a drum stick - this one is very mellow and needs significant wind to make it ring, not too loud, not too dim, just right. It also looks great.
Here is what is sounds like: Click to Listen
Here is a link to Amazon.com where you can purchase the same chime: Woodstock Dharma Cowbell