Here’s an idea that many people have enjoyed: an inverted bowl or half gourd floating on water can produce a satisfyingly deep, tone when struck with a mallet. The tone comes from the air trapped beneath the bowl, and the pitch relates to the volume of enclosed air. I found that the bowls of thin pressed wood shown here work quite nicely. I added styrofoam floats to the sides of the bowls to prevent them from tipping sideways into the water (as they otherwise sometimes do).
This audio clip starts with the long-sustaining sounds of Dansal 3. The inverted floating bowls soon enter, and later the wavery bell-like sounds of metal bowls floating right-side up.