2 days
Two nights in Oban
McCaig's Tower, a window of chocolate on the esplanade, and the distillery in town.
Oban is small enough to do on foot, which is the point. Sleep both nights in the town. The station, the distillery, and the bay are the same short walk.
Day one. Arrive and wander. Walk up to McCaig’s Tower from the station clock, then along the esplanade to the Oban Chocolate Company. You can watch the workshop through the window. Eat, and sleep.
Day two. Oban Distillery is the visit, and it sits well at the end of the day. Book it on their page. Summer days and cruise-ship days fill the way the esplanade does when a ship appears. Afterwards the town is the evening. The ferry can have the morning.
Mull is a different story. Tobermory likes its own night, and a sailing that has been looked at in daylight.