We stayed in the Oban area a couple of years ago and one thing I would recommend anyone doing is the three island cruises.
Yo can book through Caladonian McBride and it is the best day out I can remember having for a long while.
You get the Ferry from Oban ealy in the morning and sail to Mull, you then get on a coach and travel accros Mull to Fionnphort where you get on a smaller boat that takes you to Iona, after a walk round Iona you embark on the next leg of the trip and it takes about 1 hour to sail to Staffa where you can visit Fingles Cave where Mendohlson was inspired after landing and exploring Staffa you begin the journey back, although I believe they now include a trip to either Trishnish or Skye.
This trip was the best value for money i have experienced.
See
http://www.calmac.co.uk/Mull for all the uptodate info....
Enjoy
Pete