I'm surprised that nail capping hasn't been mentioned. The caps are made of vinyl and are glued (with a kitty-safe formula) onto the cat's existing nails. Once the nail caps are applied they remain in place for approximately 4-6 weeks and then will fall off with the natural growth of the cat's nails. It can be done at the vet's office or you can buy a kit at
http://www.softpaws.com/ (although I have never tried to buy them online). As with the option of declawing, it leaves your cat defenseless, so it can't go outdoors. I guess the option really depends on if the cat is comfortable with accessories, and if you (or your vet) can manage to get the caps on.
If the cat isn't scratching directly at the couch, it might might stop the cat from scratching outright until it refines control of its claws and would allow it to become an outdoor cat later on.