At that time of day on a Sunday, all of the following are within 15 minutes of your Avis drop-off location at the airport:

• Marriott Long Wharf - As the name implies, this hotel is on the water. Close to downtown tourist attractions, but also adjacent to the "Big Dig". Dinner at the Chart House (next door) is worth the price or walk into the North End a few blocks away for great Italian food.
• Marriott Copley Place - This hotel is attached to the Copley Place shopping center and is a covered walkway away from the Prudential shopping center. Many great restaurants within a few blocks.
• Marriott Cambridge - On the Charles River. Ask for a room with a view of the Boston downtown skyline. Reasonably close to Harvard Square. Not as much within walking distance as the other choices.
• Marriott's Custom House - Located in Boston's historic Custom House, this is a timeshare property. About three blocks from the Marriott Long Wharf. Adjacent to Faneuil Hall/Quincy Market (good) and the Big Dig (not so good). All suites - irregular shapes to fit the contours of this historic building.

If you want to ditch your car the night before, stay at the Hilton or Hyatt at the airport and take the shuttle to your terminal the next morning.