The ferry is expensive with a car, and will waste a few days. But car rentals on the rock are not cheap. I totally lucked out with Avis on a one way from St John's to Deer Lake for ~$500 for 10 days or so. I'd check on the price on that, because the drive is probably something you only want to do one way. We went to Twillingate, which is a really cool place to watch icebergs(though late July might be too late for it). Hiking in Gros Morne is incredible, but if you aren't up for that you can take a boat here:
https://goo.gl/maps/uLicAhk5iiDdCLA37
Which was awesome. We stayed in Rocky Harbour.