Crowds start building in France anytime from about late-April onwards, peaking from mid-June through the end of August, then start tailing off in September. So rule out the 10 or 11 weeks from mid-June through August. Weekends at the big sites are very busy from April thru June as well, since many French people visit them on nice days, and every school in France seems to schedule student trips to Paris during those spring weekends. In my experience, the best time to go (and still enjoy relatively nice weather) has been April or October. It'll also cost you less, too! Popular places will always have crowds of some size or another, but in Paris, most places have managed to figure out ways of moving people thru quickly. The exception is the Eiffel tower, where capacity's limited by the elevators, so go just before opening time to beat the lines.