Did you ever find a good spot? Not sure how much you want to ride, so take with a grain of salt anything I suggest.
There’s a Market Basket + other shops off of 117. Lots of parking available.
There’s a nice parking lot at the Norumbega conservation site; the parking lot is across the river from the boat rental spot; you get to the parking lot off of River Road.
There used to be a P&P location at Lyons Park, about 1/4 mile east of the Marriot hotel, on the carriage road parallel to Commonwealth Ave. It’s public parking, and I have used it several times this summer.