The island of Kerrera is a great day out. It sits in front of Oban and is 4 mile long.
You can get a passenger ferry (01631565333) from the North Pier, and just have lunch at the Waypoint Bar and Grill ( 01631565888), or also walk around the island.
You can also get the passenger ferry from Gallanach in Oban at the other end and walk from there. You can visit the Gylen Castle, stop at the tea room for a bite to eat and then comntiune round to the west side along the landrover track and back to the Gallanach ferry. You could also walk to the end with the Waypoint Restaurant but the walking isnt so easy.
Walk Highlands have a detailed description of the walk here.