Hertz is probably (usually) more expensive, but that is what i have used and it is a couple hundred feet across the street from the terminal with a few other companies in the "infield" as described by a previous post. Don't get on the shuttle bus, just walk - I got on the shuttle bust first time and drove around and ended up 250 feet from where I started.
Circling the island is certainly doable in an afternoon, but just be aware that if it is a weekend, you can actually run into traffic on the North Shore as you drive around.