Hi Bill-my last time in Geneva was 2003, for just an afternoon and evening after a ski trip, and I had dinner at an inexpensive Middle-Eastern place and ventured by train on a cheese pilgrimage to Gruyere, so I don't have recent recommendations for killing 6+ hours between flights. Hanging out at airports isn't necessarily fun, and I haven't been to Geneva's, so don't even know how long it would take to get into town and back for an excursion.
However, on our flight to Madrid last fall, flying from the USA to London, and then on to Madrid on a separate plane (all part of the same journey on British Airways, and never leaving Heathrow Airport or collecting our checked luggage before boarding our Madrid flight after a wait in the terminal), we did have to complete a British Landing Card, go thru Immigration (having our passports checked and confirming that we were simply in transit to Spain), then go thru Security before being allowed to venture to the gate for the Madrid flight. You'll probably still go thru Immigration even if you don't venture to downtown Geneva during your layover. Maybe you already knew that, and just anticipated going as well thru Customs (will you actually have anything to declare for Duty?) if you leave the airport; and of course, thru Security when you return to the airport for your Madrid flight. How long Immigration will take and how early the airline wants you back before your Madrid flight could cut into the actual time you'll have for seeing Geneva and having lunch. Have a great time in Espana!