A spontaneous visit to CERN was surprisingly rewarding. I managed to sneak into the fully booked French guided tour. For the next two hours, we visited two spots: first was the historical synchro-cyclotron, which surprised with a well-produced animation of its inner workings, projected directly onto its skeleton. Then we went to the building above the ATLAS detector, which (of course) could not be visited directly. Instead, we saw another presentation. A small bonus was the direct view into the control room.
Apart from the historical centre (and CERN, of course), Geneva has many nice spots. Here is a sample of two days aimlessly strolling around.