An 18km run/walk is enough to visit the entire island, but can also be broken into 2 separate trips. The 11km southern circular takes in Gylen castle built in 1582 by Clan MacDougall. The northern route is about 7-8km (including return via the same path), and leads to Hutcheson’s monument with fine views back across the harbour to Oban.
Payment is card only on the ferry (if you are the kind person that paid for me – please get in touch so I can repay you). Ferry was less than £4 – check the timetable. Gylen castle is free of charge.