My family (four of us incl. wife and 2 kids, ages 10 and 6) will be visiting the Boston area next week, renting a car to reach Portsmouth, NH for the air show and just south of Providence, RI for a DCI competition, with a stay in downtown Boston for Harborfest, etc. The itinerary has me seeking advice form locals or those who know Boston well as to the best strategy for dealing with the rental car.

The itinerary:

Fri. 6/29 evening arrival at BOS, stay at Sheraton Colonial, in Wakefield

Sat. 6/30 Attend air show at Portsmouth, NH if weather is good. If rain is forecast and Sunday's forecast is better, we'll head into Boston for the day. Second night at Sheraton Colonial, Wakefield.

Sun 7/1 If we skipped the air show on Saturday, we'll go there on Sunday. Otherwise we'll head into Boston. Sun. eve. we'll check into the Sheraton downtown Boston (Back Bay, actually, near the Prudential Center).

Mon. 7/2 Sheraton downtown Boston

Tues. 7/3 Drive to RI for the DCI show in the evening, stay overnight at Sheraton Providence airport.

Wed. 7/4 Back to the Sheraton downtown Boston

Thurs. 7/5 depart from BOS airport.

So here's the dilemma: I need the rental car for the trip to Portsmouth and the trip to Rhode Island, but I definitely don't want to pay the downtown parking rates for the nights at the Sheraton downtown Boston, and won't need the car for those days, either. I currently plan to return the car to the airport on Sunday, then take the T from the airport downtown to check in there. Then we plan to rent a car from a downtown rental location near the Sheraton, drive to RI, and return the car to a downtown Boston location for our 7/4 stay. Then we'll take the T to the airport for our departure.

A weekly rental would cost about the same, or maybe even a little less, and would avoid the hassle of getting back to the airport to return the car on Sunday and take the T downtown. But I can't figure out how to park the car inexpensively and in an easy-to-reach location during that time.

I'm renting with National, and it seems the downtown rental car offices are closed on Sunday so I haven't yet found a good option for returning the car that day to a location more convenient to the hotel than the airport.

If I have to return the car to the airport on Sunday I may consider driving the family directly to the hotel downtown and letting them check in and hit the pool while I drive out to BOS to return the car and take the T back to the hotel. What should I plan for the round trip driving from the hotel to BOS and then taking the T back to the hotel? A couple of hours? We may stop at Wal-Mart on the way to Portsmouth to get 4 cheap folding chairs for the air show, and we might want to hold onto them for the Boston Pops and fireworks on the 4th. In that case, I'd sure like to avoid having to schlep the chairs plus our luggage on the T.

If there's an obvious improvement to the above rental car shuffle, I'd appreciate any advice.

Thanks.