This is a very enjoyable bike-ride, 26 miles there and back. Or you can take a bus from Dunoon as far as Castle Toward (there are several a day) and then walk from Castle Toward to Inverchaolain, which is five and a half miles each way.
The trip is almost entirely along the lochside (Firth and then Loch Striven), so pretty flat most of the way, and usually very little traffic (it comes to a dead end shortly after Inverchaolain). The coastline is beautiful, and there are lots of birds and animals to see.
Here’s a map centred on Castle Toward, which is roughly the half-way point of the journey from Dunoon to Inverchaolain.
There is another way of getting to Inverchaolain from Dunoon – an attractive cross-country route of nearly six miles known as ‘the coffin route’. You will need your Ordnance Survey map for this, and wear good shoes and waterproofs. Here is a map of the route, centred at the starting point: the bottom of the Glen Kin forestry track. Follow the track SSW up the glen, past the Glenkin farm (now a bunkhouse) to the head of the glen. The track will eventually turn and head back down the glen, but we keep going SW towards the pass ahead of us, Bealach na Srèine (‘pass of the bridle’, or ‘of the stripe’). At the head of the pass (NS1177) we look down into Inverchaolain Glen and head down the hill, keeping the Inverchaolain Burn to our left when we meet it. Half way down we come to the forestry track on the right (NW) side of the burn. Follow that down to the farm of Stronyaraig (Sròn Dhearg ‘red nose or promontory’) and then down to the shore road. Turning left we shortly arrive at Inverchaolain.
You could walk back the way you came, over the hill. Or you could take the shore road south to Castle Toward (five and a half miles) and catch the bus back to Dunoon. Check the Ardentinny-Toward bus timetables before you go!