I am planning to rent a convertible when I am in Toronto this September, and drive around with the top down. My worry is whether the climate will be suitable for this?
I can't predict the weather three months out. However, I can point you at average temperatures for Toronto in September. It looks like it could be hit-or-miss, depending on your tolerance for cold, and that the earlier in the month, the better your chances, especially during the warmest part of the day. Note also the average rainfall of 2.9" for the month.
I can also link to this chart from the Canadian seasonal prediction system, which appears to show a broad general trend toward temperatures above normal during this time. This is a broad trend, and could be completely inaccurate, or just not representative of any particular day you're there.