Visiting Halifax and St. John's

I would like to combine Halifax Nova Scotia and St. John's Newfoundland into one trip. I would be flying out of the Washington DC area, USA. What's the best/most economical way to do this?

Would it be possible to book a flight from Washington to St. John's, for example, with a multi-day stopover in Halifax - to allow for 6 or 7 nights to rent a car and explore Nova Scotia/Cape Breton - before continuing on to St. John's? (I don't think I would need a car in the cities of Halifax or St. John's).

Or is there a better way? (I know there is a 12-hour summer car ferry from Sydney in Nova Scotia to Argentia in Newfoundland - but that seems like it would take too much time and not save much cost).

thanks.