Consider an image-based, interactive captcha using drag and drop.
Show a map with a bee on it. Please guide this bee to the flower, passing the pond, the bear, and the scarecrow. Now bring the bear to the beehive; you would generate a hash string based on the sequence of visits. This may involve multiple back-and-forths, which is probably bad, but fairly brainless and robust, from what it seems, as little thought as I have given it.
Show a map with a bee on it. Please guide this bee to the flower, passing the pond, the bear, and the scarecrow. Now bring the bear to the beehive; you would generate a hash string based on the sequence of visits. This may involve multiple back-and-forths, which is probably bad, but fairly brainless and robust, from what it seems, as little thought as I have given it.