France is a very safe country and you'll feel secure wherever you go.
Have to agree that Rennes is not that interesting. You mention Midi Pyrenees, but haven't listed any of the towns there. Is Toulouse one of the options? If so, give this city some serious consideration. It's a beautiful place, plenty to do, and is a city full of young people.
Not sure about Nice, but I guess you've got the beach in summer.
While Paris will be lovely remember that your host family will most likely be in the suburbs, well out of the city centre. But there will still be plenty to do there.
Another city worth considering is Lyon.