I am looking at hiring a car in downtown Boston as I will have already been there a week and heading to Freeport Maine on a Sunday Morning early to avoid crowds. Any advice please, we are from Australia.

When will you be doing this? You won't have a problem with traffic on Sunday morning unless there is some sort of fundraising march or festival. These are not likely in the winter and almost constant in the summer.

You will need to determine from the car rental site whether in fact the location you want to
rent from is open "early" Sunday morning. Some of those in the Financial District don't open at all on Sundays, and I think those in Park Square are likely to be open, but only the company can tell you for sure.

We picked up and dropped off our rental car from National Car Rental on Atlantic Avenue , near the Aquarium. It has easy access to the interstate highway system. When we returned it, we walked up the street to the Boston Harbor hotel and took a water taxi to Logan airport, easy too and a traffic way to get there.

Agree with National/Alamo near the Aquarium, and they usually have good rates for Aussies that include the insurances and fees. But what part of Boston will you be staying in?

And Atlantic Avenue is open at 8 am on Sundays, according to their website.

Where are you staying? I'd look for a rental car office nearby. In addition to the waterfront and Park Square, most agencies have offices in Back Bay/Copley Square. It is easy to exit the city and go North from there.

The Hertz in Park Square and the Avis in the Back Bay (100 Clarendon) are easy to get to from the subway and are convenient to the highway. They would probably be my first choices. National sounds okay too, but I prefer to rent from Avis and Hertz.

Our rate for two months was National was way way less than the others.