I'm relatively proficient at the violin and viola (last truly classical pieces I played were Carmen Fantasie and Sibelius, but the former was a bit over my head...now I mostly do electronica, jazz, and country shut the fuck up, with them), and am decent at four string mandolin/mandocello. I also try to play classical guitar but I don't know any of the theory and I am pretty lousy in general. This is applicable to piano as well.
EDIT: Oh yeah, my grandfather taught me the er-ching, but that is a shit instrument IMHO so fuck that sorry grandpa.