I travel to Switzerland and Europe at least once a month. Travelling between European cities I usually take the train - and if you are willing to extend your borders then you have Milan, Venice, the lakes all within easy reach of Geneva.
With no disrespect to the Swiss, but you will be pushed to fill 12 days, unless you are happy to find a small village to hide away. Zurich and Geneva are usually asleep by 9 / 10pm...
You do not mention your likes, dislikes or interests - but if you like visually beautiful scenery, fresh air, cities that go to sleep early and fabulous service in some of the worlds best hotels, you are about to embark on a magnificent 12 days what I would describe as relaxed beauty...
Make sure you take a full wallet, Switzerland can be very expensive. Even the taxi drivers wear tailored suits and gold Rolex's.....