I visited the outer Hebrides a couple of years ago. We Lewis and Harris, which are really the same island. I was on a tour--a small one of about 14 people and we traveled to Ullapool and caught the ferry to Stornoway. We saw the Butt and then headed south along the western coast. We stayed near Dun Carloway for a couple of nights and then went to Tarbert. We were on a walking tour, but also saw the Broch, the Viking Mill, a Black House, the Callanish Stones, visited a weaver, and saw St. Clements on the Southeastern tip of Harris. We also heard a fantastic talk on St. Kilda in our hotel in Tarbert one night. We took the early morning ferry from Tarbert to Uig on Skye and we were back in Inverness that night. If you don't have a tour, get a car. It was a wonderful trip. I highly recommend it. You need to walk on the beaches as well. I don't know of any hop on hop off tours, but there are a lot companies that will do very individual tours if you're willing to pay.