Loch Lomond is a freshwater Scottish loch which crosses the Highland Boundary Fault. It is the largest inland stretch of water in Great Britain by surface area.
Hiking is also a good idea. You could for example go to Rowardennan and then hike up Ben Lomond. The long-distance West Highland Way also runs along the eastern shore on its way from Milingavie (near Glasgow) to Fort William.
Buses run along the Loch, but not that frequently . You should check the departure-times first, as timetables are not usually available at the stops. The National Park Authority publishes a timetable of all the buses and ferries which may be available as a booklet locally.