For a one-button option, I think I'd like something like a knob pointing to the current state (like a label saying ON), which when clicked would point to the ot

For a one-button option, I think I'd like something like a knob pointing to the current state (like a label saying ON), which when clicked would point to the other state (OFF). Both labels should be visible at all times. I think people may understand knobs. I'm an absolute fan of skeuomorphism but you may be able to design something more abstract (but that's a slippery slope so hopefully not).