Many Canadians don't think twice about jumping into their cars and driving across the border to the USA. Canadian car insurance is valid in the states however, you may wish to check with your insurance company to find out whether your liability coverage would be adequate there given the higher amounts awarded in personal injury cases.
You'll also want to make sure that you have health insurance even if you are only heading over for the day. OHIP may cover some of you costs if you are hospitalized in the US but it certainly won't pay enough. The hospitals in the US accept cash and credit cards as payment if you don't have insurance. The amounts come as a shock to Canadians who are not accustomed to seeing what their health care costs.
If you are renting a car in America be sure to check what coverage your own Canadian policy provides you while renting, and consider topping it up.
If you travel outside the USA, to Mexico or Cuba for example, you will need to have local insurance. Also note that you will be subject to the laws of the country that you are in should you be involved in an accident while you are driving, and in some countries you will be denied exit until all investigations and prosecution is complete.