Be sure to obtain unlimited mileage car rental, since distances in the USA and Canada are immense. If you plan to fly out to one airport and back from a different airport, then check for any drop off charge for the car, which can be very high. Sometimes you can drop the car at another location within California or Florida without incurring a drop-off charge. Also check whether driving is restricted to certain states, and whether the car can be driven into Canada if this is relevant. As well as the size of the car, you might also consider upgrading from a two door car to a four door car.
Car rental is usually much cheaper for cars hired from within the USA rather than within Canada.
Rental from within Florida and California is usually much cheaper than from within the other states. It is very difficult to obtain unlimited mileage in Canada. The car rental voucher usually only covers the basic rental cost. You must pay all the other costs locally. There are also restrictions and costs involved with young drivers.