i would say two lenses one for a day with the family or race events etc
where zoom isn't really necessary plus you will need to open the shutter a little bit longer when there is low light etc and the Image Stability is great for this!
canon 17-85mm IS its a great lens
and one for the far shots wildlife being in the cheap seats at the Grand Prix etc.
canon 70-300mm is lens
both great lenses!!