Go south from Oban and visit the Kilmartin museum (great cakes, screw the exhibits, subjects are all dead in anyway). En-route, turn in to Ardfern and admire the Craignish peninsula. Have a beer at the Galley of Lorn hotel. Sit outside, admire the loch and swat the midges. Then, hop on your bike and go further south, up the hill and take a photo looking back at where you've just been. That should be enough of a trip for one looooong day.
Cake shop stops on the way. 1st stop, Kilmelford café and shop. Closed on Mondays but open for the rest.
2nd stop. Crafty Kitchen in Ardfern. Scones, butties etc. Supposedly vegetarian but you can order a bacon roll. Opposite that is the Galley or Lorn. Fancy meals, pub food or just beer outside on the benches.
3rd stop, Kilmartin museum and café. Scones, cakes, beer, cider and good coffee.
How fit are you? There are some serious hills between these two points.