If you want to get the most out of your trip, then your first stop should be the tourism office, which is on the front pier next to the ferry terminals in Oban. Pick up brochures and take them back to where you are staying, and make a concerted effort to find out what you want to do and see when it is open. There are things well worth visiting outside of Oban, and short ferry rides can get you there. I can't put my finger right now on the castle we went to, but we got the ferry to the island, took a short miniature train trip to the first castle, and then walked to the next castle before it started raining.
Locals told us the castle was a 15-minute walk - but an hour later, we were still walking. Their joke, do you think? Anyway, we got picked up by a Lord of the castle and he gave us a lift right into the castle in his car! All around the castle there were pictures of him with the Queen, etc. It made for a very amusing visit. The castle was amazing, the people great, and there was a cafe that served nice food. So look around - a little off the beaten track, but if you've got walking boots or a car, then make the effort, because this area is just filled with amazing things.