In the Phineas and Ferb season 1 episode “Mom's birthday”, we find that Candace plays several musical instruments: bongos, bugle, bassoon, bagpipe, bass, banjo, balalaika. (She also considered taking blender lessons in “Flop starz”.)
But I don't recall Candace playing any musical instrument in later episodes. She has lots of opportunities, because the show is full of music, and the other main characters do play instruments. Despite that, it seems like Candace always just sings. Am I missing something? Is there any other episode where Candace plays a musical instrument? Or did she give that up completely when her instruments disappeared?