Chopsticks? Woodblocks!
From the Wikipedia article on [ Piano Sonata No. 2 by Charles Ives ]:
… The piece demonstrates Ives' experimental tendencies well: much of it is written without barlines, the harmonies are advanced, and in the second movement, there is a cluster chord created by depressing the piano's keys with a 14 3/4 inch long piece of wood.
IINM prepared piano is not a playing technique but a piano internally modified, like putting something between strings.