You don't need a visa to enter the US, but you will need an ESTA ($14). It's best to get one online in advance, which takes just a few minutes:
https://esta.cbp.dhs.gov/esta/
Crossing the border with a rental depends on the rental company you choose. Obviously you should check when booking the car, that you're able to do so. Hertz are reportedly OK with it:
http://car-rental.hertz.ca/faqs/how-can-i-find-out-more-about-driving-between-usa-and-canada