Music wall from the blog Pre-K + K Sharing
Have a hootenanny
Hang up muffin tins, old pots and their lids to make an outdoor instrument amphitheater, like this music wall idea from Pre-K + K Sharing. More ideas: Fill bottles with beans to make a rattle. Collect some sticks or use kitchen utensils to bang on pans. The sound of raindrops falling onto pots and bottles will complement your kiddie cacophony.
Next up: a mud bath