Fort L'Ecluse years ago, on another hookey trip, and wanted to check it out again. However, this time they'd completely revamped the fort, made it into a museum, and given access to the upper fort (Fort Superieur) 1000 steps in the air. While I hadn't expected a Washington Monument level climb, it was totally worth it for the views (of Mont Blanc, the Rhone, and the Vuache) which happened to be on the most perfect day I'd seen in Geneva for quite a while. Anyway, enjoy.
For those curious, the fort is a drive about 20 minutes southwest of CERN (just between Meyrin and Saint-Genis-Pouilly, straddling the border), at a "notch" in the Jura just below Collonges, which looks like this (thanks, Google):
So it gives fantastic views towards the Alps to the east, especially Mont Blanc, which features prominently in a few of the photos.