I've always liked
Penny. Ever since the days of Inspector Gadget cartoons! :) I'm not really a fan of
Penelope either, but I would prefer a fuller name than "just"
Penny.
Peninnah works well. I've never heard it. My first inclination would be to pronounce it peh-NINE-uh. But with the two n's, I'd default to peh-NIN-uh.
I don't think
Penny is in danger of being the next
Maddy.
I know of one
Penelope - who only goes by
Penelope, not
Penny.