Well, if you decide to wander the beaten track from Oban, you can get a bus or hire a car and get to Inveraray. It's a pretty little town on the edge of the estuary which has not only a few little things such as the farm park, the tower, and some nice walks, BUT also a castle.
When you get off of the coach or park your car, you will see the castle from wherever you stand. It’s quite a spectacular one. If travelling by foot, go to the front where you see the statue overlooking the water, and where the hotel is, and you will see the signpost for the main drive to the castle. If walking, it’s a 10-minute walk up the main drive to where you buy your tickets to go in. You'll find an armoury and turrets - everything you need to have in the essential castle. But the surroundings are just superb. It costs £5.30 per adult, and £3.30 for children. Well worth it, though.
There are several ATMs in the town, so if you need money, then find one beforehand, because not everywhere takes all cards.