We took the ferry from Aberdeen to Shetland. We travelled up to the very north island Unst via Yell. If you pay for the ferry to Yell and go straight on to Unst the same day, that leg us free. The return ferries are also free. So it only cost us around £20 to get to the most northerly part of the British Isles from the main island. There are a few sites on the islands but wild camping is no issue. Make sure you stock up at Tesco in Lerwick on route as shops are few and far between. We then caught the ferry back to Orkney, then back to Scotland. It cost us around £485 for the ferry return trip including overnight cabin to Shetland. We spent around two weeks on the islands and needed it!