I can't help you on the oyster part but I highly recommend getting in the car and going for a drive! I've never been to Darnley but the north shore is pretty nice in general. (Ok I just googled it and it looks very cool so I say yes go for it!) I'd suggest doing a loop heading up towards Prince Edward Island National Park/Stanhope area first and making your way along the coast to Darnley for the sunset.