On hiring the car hire check that you are able to drive on the Swiss motorway.  The cars usually have a sticker in the windscreen giving you entitlement to drive on the Swiss motorway (otherwise on returning to Geneva you will either have to pay about 100 Swiss Francs or go round the long route staying in France)

Remember to return the car to the correct side of the airport & to the correct hire car park.  It is a good idea to remember exactly how to get to the car park on return the car hire.  Also allow plenty of time to refill the car with petrol before you hand the car back.  Again it can be difficult to find a garage close to the airport.