I can't imagine you'd run into any problems (other than the completely random chance of a cranky customs agent which can happen anywhere).

Most of the border crossings between the US and Canada on the East Coast are relatively mellow. Just have the address for where you'll be staying handy for the forms.

And make sure your rental car's insurance is good in the U.S. and you can take the car out of Canada.

Come quick, the snow is melting pretty fast this year!