Thanks once again to an article in Dwell, I've found Bemz, a company with a brilliantly simple product.
Bemz produces custom slip covers for IKEA couches, including discontinued models. This is a cost-effective way to renew an old sofa, or to add color at low cost.
Here is our stuga's Karlstad sofa in one of their crazy-bold patterns. Don't worry, this is just for illustrative purposes. Sooz wouldn't let me do anything like this!
Even a high-quality fabric like this costs only $159. Simpler colours and patterns are as low as $59. It would be easy to carry a slipcover (or two) out to the island. Changing out our footstool every so often would make a significant difference to the interior of our house. Perhaps a bright yellow during the darker months? A floral pattern for the spring? It's fun to think about.